There has been a lot of talk about Iran making a sudden dash for the bomb. With its thousands of gas centrifuges, and its tons of enriched uranium, Iran might be able to make a bomb’s worth of nuclear fuel before any other country, such as the United States, could intervene to stop it—that is the fear. In September, Israel’s Prime Minister Benjamin Netanyahu went so far as to display a bomb diagram during a speech at the United Nations; then he drew a red line on it showing when the dash might occur. He said it could be as early as this spring.

It is surprising that this chain of events has gained such currency, because it is not likely to happen. Iran, in fact, doesn’t seem ready to dash for the bomb. It is playing a much longer game, all the more menacing because more likely to succeed.

Iran’s strategy is simple: to be able to make a nuclear weapon at an acceptable cost. To achieve that, Iran must avoid taking any drastic step that would trigger a war. In a shoot-out with the United States, the ayatollahs would risk their survival—a large cost indeed. Iran’s leaders have stayed just beneath the line of intolerable provocation, where they have nurtured their nuclear prowess with small interference. Of course, they must also keep the pain from sanctions low enough to avoid revolt. They are succeeding there as well. So their strategy has been working. Its success is only one of the reasons why Iran probably won’t dash for the bomb anytime soon.

I. Who wants only one bomb?

Such a dash is unlikely for a second reason: no country wants only one bomb. That is especially true of the type of bomb Iran has been trying to develop. It employs the principle of implosion, and would have to be tested. The United States was obliged to test its implosion bomb in 1945 before dropping “Fat Boy” on Nagasaki. Iran too would be obliged to test its implosion design, in order to find out whether it worked, and to let the rest of the world know it worked. Otherwise, there would be no effect of nuclear deterrence, which is the reason for getting the bomb in the first place. Thus, a dash to produce one bomb’s worth of fuel—a possibility that has uncorked a small torrent of ink estimating how long it would take—would cross the finish line with mainly test data.

But the most important thing about the dash is that it probably would be detected. The head of U.S. intelligence, James Clapper, assured a committee of Congress in March that "Iran could not divert … material and produce a weapon-worth of … [uranium] before this activity is discovered." By “material,” Clapper meant the stock of uranium Iran already has enriched. Most of it is “low-enriched,” meaning two thirds of the way to weapon-grade. A small amount is “medium-enriched,” meaning ninety percent of the way. The former is suitable for power reactors; the latter for research reactors. To make a dash, Iran would have to start with this enriched material; to start with natural, un-enriched uranium would take so long as to be impractical. But there is a catch: all the enriched uranium is regularly checked by U.N. inspectors. Within a few weeks at most, they would be likely to detect its diversion, and alert the world to the danger.

The result would be a perfect storm, politically. There would be tremendous pressure on governments to act. Neither Israel, nor the United States, nor Europe could afford not to. Nor could Iran’s neighbors in the Gulf fail to urge these countries on. Russia and China, Iran’s international defenders, would try to come to the rescue, but they would be hobbled by their client’s illegality—Iran would be in flagrant breach of its obligations under the Nuclear Nonproliferation Treaty, which requires continuous inspection of enriched uranium and forbids its use for any but peaceful ends. Iran could wind up facing the very war it had been trying to avoid.

II. Iran’s “dash” would be more like a crawl

That war could easily start before the dash succeeded. To produce bomb-grade uranium, Iran would have to enrich its stockpile further. That means passing it again through its centrifuges. Unluckily for Iran, but luckily for just about everybody else, Iran’s present generation of centrifuges would take a long time to do that. They are surprisingly inefficient.

Iran’s largest known enrichment site is near Natanz, about 160 miles south of Tehran. It features a cavernous underground space, shielded by concrete walls, that is home to some 9,000 rapidly spinning centrifuge machines. Let’s look at how long it would take these machines to raise Iran’s uranium stockpile to weapon-grade.

Only a small amount of work would be needed, in theory, if one started with the medium-enriched uranium. If it were fed it into the 9,000 machines in ideal fashion, and the production process proceeded without a hitch, and if the centrifuges achieved the same efficiency as they have so far, the minimum theoretical time could be as low as a few weeks.

That estimate, however, comes from a standard industry model that hardly fits Iran. The model calculates automatically how much work would be needed to raise the enrichment of a given amount of uranium by a given percentage. But the model is for a commercially mature enrichment plant operating under ideal conditions. Iran’s first generation set-up at Natanz is far from that. It has frequent breakdowns, made worse by sabotage. The real-world time for enrichment at Natanz is likely to be twice as long. But regardless of these estimates, there is a big problem. Iran has only enough of this medium-enriched material for one bomb. To fuel even a small arsenal of five bombs, Iran would need to amass far more, which would take another three years at its present rate of production.

Could Iran start with its larger stock of low-enriched uranium? It has enough now to fuel five to six bombs, a plausible number for a beginner’s arsenal. How long would it take to raise that uranium to weapon-grade? The theoretically minimum time, again using the industry model for a mature, commercial plant, would be about eight months. The real-world time would certainly be longer. It should be obvious that such a production run won’t happen. It would not be a dash, or even a jog. It would be a crawl. And in plain view. Long before Iran could fuel even five bombs, precision-guided munitions could be streaking down. Although Natanz is buried deep, U.S. ordnance would be able to reach the centrifuges.

Iran also has a second, newer enrichment plant at a site called Fordow. It was built in secret, tunneled into a mountain, protected by blast doors, and surrounded by anti-aircraft batteries. Although a daunting target for aircraft, it would be less threatening than Natanz in a sprint for the bomb. It has only a third as many centrifuges. So it would need three times as long to process the same quantity of uranium. It doesn’t really work for a dash. Its main strength is air defense.

III. The weaponization question

There is one last matter that will determine whether Iran might try to get a bomb in a hurry. It is called “weaponization”—that is, making the actual components of a bomb. In addition to making the uranium core, Iran would have to design, manufacture and test the other components that make an implosion bomb work. That would include high explosives machined carefully into special shapes, a firing set containing precision switches, and many other parts of precise dimension.

These components are true pacing items. They would have to be in hand before a bomb could be assembled. There would be no point in making a dash until they were ready. Why risk a war for weapon-grade fuel if there is no way to detonate it? It would be like holding up a store with bullets and no gun. It would never happen.

Despite its clear importance, Iran’s exact progress in weaponization is a mystery. By nature the work is secret. And it happens on a laboratory scale, which makes it difficult to detect. It is also illegal—it would violate Iran’s pledges under the Nuclear Nonproliferation Treaty. Nevertheless, a number of intelligence agencies have detected Iran’s progress in this domain, and have handed the results to the U.N. inspectors. They, in turn, have published details of weapon-related work in their reports. Damning it certainly is. Iran has experimented with just about every technology needed to design and test a bomb, and practically none of it fits a civilian program. Iran won’t comment on the data; it rejects it as a fabrication.

We are left to guess how many bomb components Iran might have perfected, and thus whether it is even capable of making a bomb. This is a huge gap in our knowledge. It is big enough to make all the predictions about a dash look rather empty.

IV. Working secretly would be better…but it would still be a crawl

The main thing we do know is that Iran has been doing its weaponizing in secret. And that raises an interesting question. Why not do the dash that way too? After all, if Iran is going to have to feed the low-enriched uranium back through the centrifuges to raise its potency to weapon-grade, and if that is going to take eight months, why do it at a known site? Why not do that in secret too? It would be an illegal act under the Nuclear Non-proliferation Treaty, as explained above, but so is making bomb parts. If you are obliged to break the rules for some of the game, why not break them for all of it?

That is precisely what Iran is expected to do. U.S. intelligence agencies predict that Iran will probably perform its last enrichment step at a secret site. That would avoid an attack on the site if it could remain hidden. But even so, things would not be easy. Let’s look at what such a site would have to hold.

Suppose Iran wanted to enrich five bombs’ worth of uranium to weapon-grade in about three months, starting with its low-enriched uranium stockpile. That would require about 24,000 of its present-generation centrifuges. And this number is a theoretical minimum taken from the industry model—in the real world it would take more.

The problem is that this number is twice as many as the 12,000 Iran has managed to install so far at Natanz, its largest site. (Only 9,000 of the 12,000 are operating). And Iran has needed more than five years to produce and install that many. Where would Iran suddenly get 24,000 more machines? Could it make them? Only with a production level many times more robust than seen so far. But even if it could make them, it would have to hide their manufacture, hide the site where they were installed, and keep it all secret until the moment it was ready to divert the low-enriched uranium. Then, after the diversion, it would have to hide their operation. That is a lot of hiding, especially after a diversion. Intelligence agencies would be frantic to find the missing uranium.

Is there a way to get by with fewer than 24,000? There is, but it doesn’t help much. The number could be halved, to about 12,000, but that would oblige Iran to double the time the centrifuges would have to run—to at least six months. The relationship is unforgivably linear. There’s no getting around it. Each machine can do only a small, incremental amount of enrichment. If you take away half of the machines you need twice as long to do the same amount of work. And let’s not forget that these three or six month periods are theoretical minimums—the real time would be longer. Four months or seven months—depending on the number of centrifuges—would be more realistic. That is a long time to hide a site when everybody is looking for it.

To make matters worse, Iran still would be a step away from a nuclear weapon. It would be necessary to convert the uranium hexafluoride gas coming out of the centrifuges to uranium metal, refine and cast the metal, machine it to size, and assemble the devices. How long would that take for five bombs? One can only guess. Perhaps two or three weeks for the first, and less for each successor? This means that counting from the moment of the diversion, the work to produce five bombs could consume half a year or longer.

So even at a secret site we have no dash, only a vigorous crawl. It doesn’t seem a winning hand. The political storm set off by the diversion would not abate as long as the uranium was missing. There would probably be an ultimatum demanding that Iran allow it to be inspected. That, in turn, could produce a blockade, or bombing, or a wider war that could threaten the regime. Like their neighbor Saddam Hussein’s decision to invade Kuwait, the ayatollahs would be inviting hostilities before having a nuclear deterrent. Why risk it?

V. The long game is the smart game

It would be smarter to play the long game, which Iran seems to be doing. Iran can wait until it has all the ingredients in place for a small arsenal, and until it can make that arsenal in the blink of an eye. Then, Iran would have a real chance of success. How might that happen?

First, let’s consider the uranium fuel the bombs would need. Iran would have to be able to enrich five bombs’ worth to weapon-grade, and do it in a hurry. We have seen that it would be unlikely to start with low-enriched uranium. That would take too long. It would be much better to start with medium-enriched uranium. Iran is now producing slightly more than 14kg of this material per month. At that rate, it would not be before April 2016 that five bombs’ worth would be in hand. That date may seem a bit distant, even for a long game. It would give sanctions another three years to shrink the economy and stir unrest.

To advance the date, Iran could use its idle capacity. It is operating only a fourth of the centrifuges it recently installed at its new enrichment plant at Fordow. At full bore, the plant could more than triple Iran’s production rate to some 44kg of medium-enriched uranium per month. Whenever Iran starts producing at that rate—late this summer would be feasible—it would take less than a year to have enough for five bombs.

That would seem a logical next step, but there is more at stake than physics. The step would mightily alarm Israel and the West. Iran has no civilian need for more medium-enriched uranium. It has made enough already to fuel its small research reactor, which was Iran’s only excuse for enriching to such a level in the first place. Making more of this potent material would point only one way—toward weapons. Iran seems to be hesitating before this step, trying to gauge the cost.

Let us suppose, however, that Iran takes it, and finds itself next year with enough medium-enriched uranium for five bombs. What would Iran need to enrich it further to weapon-grade? The answer is surprising. To do it in a month at a secret site, Iran would have to employ at least 22,000 additional centrifuges. To do it in two months it would need at least 11,000. These are, again, theoretical numbers, modeled on a mature enrichment plant operating under ideal conditions. Iran’s first generation set-up would certainly take longer, or require more machines. And to these production times one would have to add the time for weaponization. The uranium gas would have to be processed into metallic bomb parts, as explained above. That would take more time, and Iran would have to produce these centrifuges in secret and install them at a secret site.

So even with medium-enriched material, the dash is looking long. In the real, not theoretical world it would never be less than two to three months. More likely, it would consume four to five. And the clock would start ticking when the inspectors discovered the material was missing. They might not discover it within the first two weeks, but they would be highly likely to discover it within three to four weeks. That could leave a couple of months at least of reaction time, and probably more, before Iran could field an arsenal. There might be more time still if Iran ran into snags when enriching to weapon-grade, or when processing the uranium into bomb parts. Iran’s scientists would be doing both for the first time; they couldn’t rule out surprises. Once again, Iran would have to risk the possibility of war before a deterrent was ready.

So what can we conclude so far? It seems that a dash isn’t practical yet. Iran’s centrifuges are just too slow. Iran would have to risk war before having a deterrent in place, regardless of whether the dash happens at a known site or a secret one. The odds are against it.

VI. Iran could bluff...

At this point we have to take a short detour, to consider a bluff. Could Iran try to make a single bomb, set it off, and claim to have more? Could it achieve nuclear deterrence that way? This possibility seems to preoccupy those who worry about a dash.

Here is how it might happen. Iran would start feeding the medium-enriched uranium into the 9,000 centrifuges at Natanz, or into a secret plant with an equivalent number of machines. Then, Iran would deny entry to the U.N. inspectors, using some pretext or other, at their next visit, which could be two or three weeks after the feeding begins. During that two or three weeks Iran would enrich one bomb’s worth up to weapon-grade. That is, it could do so theoretically, according to the industry model of enrichment times for an ideal commercial plant. Whether it could do so in the real world is unknown, but at least Iran would have a theoretical chance to produce a bomb’s worth of fuel before being detected.

How much should we worry about that? Such a sprint to a bomb can’t be ignored, but there are reasons why it probably won’t happen. The first has been described above. Iran, like other countries before it, would want more than one weapon. Its implosion design will require a test. The second reason is that enriching a bomb’s worth of fuel to weapon-grade in a few weeks leaves out something: weaponization. After getting the bomb’s worth of uranium gas, Iran would have to process it to metal, cast it, machine it into bomb parts, assemble an implosion device, and then detonate it at a test site—probably underground—prepared in advance. All this will take time. And Iran will be doing it for the first time. Even Iran could not be certain how long it would take. If Iran locked out the inspectors, news of that event would be likely to come before any test. If so, Iran could get warnings from the United States and other powers to let the inspectors back in. Would Iran then go ahead and test its single bomb, hoping no one would attack afterward? It is impossible to know. But it would be a gamble. This scenario, like the others, would risk war before a real deterrent was ready. And what if the test were a fizzle, or a dud?

A third reason why this probably wouldn’t happen is that history is against it. No country has joined the Nuclear Nonproliferation Treaty and then built a bomb by making a dash with diverted material. Libya, Iraq and Brazil all had bomb efforts while belonging to the treaty, but they mounted “parallel programs.” That is, secret programs that operated in parallel to their known program. South Africa, India, Pakistan and Israel actually made bombs at hidden sites, but they didn’t join the treaty. North Korea joined the treaty in an ambiguous fashion, but then left it before bringing its bomb quest to fruition. Saddam Hussein’s Iraq did divert enriched uranium to make a dash, but only after it was clear the first Gulf War was coming. By then it was too late. Absent the war, Iraq probably wouldn’t have done it.

In addition to all these reasons for not making a dash there is still another, and it is perhaps the most weighty. It is simply that a dash wouldn’t give Iran what it really seems to want. And that would be an unquestioned nuclear deterrent, obtained at acceptable cost. Why chance war to get one weapon, or a few weapons, that may work or may not? Wouldn’t it be better to follow the example of Israel, India and Pakistan, which came to the bomb late, but got there without armed conflict, and are accepted now as true nuclear powers?

The great advantage of the long game is that it provides a chance to do things right. Iran is going unmistakably in that direction. It is already experimenting with better centrifuges. The new ones are reported to enrich uranium three to five times faster than the present ones. Several hundred of these machines have arrived at Natanz already, and Iran predicts it will install three thousand “in the near future.” It may take a while before Iran can perfect these machines, but when it does, it will have come a huge distance forward.

Enrichment is also increasing. Sometime next year, Iran will amass enough low-enriched uranium to fuel seven bombs, and if it activates its idle capacity at Fordow, enough medium-enriched uranium to fuel five more. A dozen bombs begin to resemble a credible nuclear force.

And Iran is creating a second path to nuclear weapon fuel. It is building an unusual nuclear reactor that is too small to make electricity, but larger than needed for research. It will yield enough plutonium to fuel two nuclear weapons each year, and could also produce tritium, which is used to multiply the power of implosion-type fission bombs. This type of “heavy water” reactor is rare—there are few in the world. India happens to have one, and so does Pakistan, and so does Israel. They are all fueling bombs today. (This should tell us something about Iran’s intentions.) The long game would allow the reactor to be finished. A war would probably turn its above-ground location to rubble.

VII. Iran’s comfort zone is Israel’s danger zone

So roughly two years from now, we could see a new situation. Iran could have a hefty stockpile of medium enriched uranium; it could be operating several thousand of its potent new centrifuges; it could be operating thousands more of its old centrifuges—which might enable a dash even with low-enriched uranium—and its heavy water reactor could be ready to come online. It may have entered at last the “zone of immunity” Israel has been warning about. By zone of immunity, Israel means that Iran’s power to make nuclear weapons would no longer be vulnerable to air attack.

That status is exactly what Iran seems to be striving for. What Israel dreads as a danger zone, Iran would welcome as a comfort zone. Throughout the Islamic Republic, a growing number of workshops would be busy forming the parts for thousands of centrifuges, of at least two different varieties, at locations unknown to international inspectors. Iran would also house a growing number of storage cylinders, containing Iran’s low-enriched and medium-enriched uranium. Iran would have reported their whereabouts to inspectors, but the cylinders are moveable. They could be loaded on trucks and driven away in the first hour of any attack.

And what about the dash, our original question? If Iran fed its new centrifuges with its new stock of medium-enriched uranium, it could shave enrichment times to the bone. Instead of two months or more to fuel five bombs with 9,000 or so old-style centrifuges, the theoretical time for enrichment with the new ones would shrink to two to three weeks. Of course, weaponization would take additional time, but there as well Iran might have pushed ahead.

According to Yukiya Amano, head of the U.N. agency that inspects Iran, the agency has evidence that Iran is still working to perfect nuclear weapon components. He told the Associated Press in March that, according to the evidence, Iran was working on “the development of nuclear explosive devices in the past and now.” If this is true, Iran would surely benefit from having additional time for that development. Playing the long game affords more opportunity to get the bomb components right, and more confidence they would work when needed.

VIII. Time favors Iran

We are left with this conclusion: a dash is complicated. It raises questions beyond enriching uranium. Perhaps the real question is: whose side is time on? The answer seems to be: Iran’s. As its nuclear program grows, the program’s products will be harder to keep track of, and thus harder to attack. This is bound to create an aura of inevitability. Iran can reasonably hope that when its program is fully mature, the world will decide that an aerial attack won’t be adequate, that all-out war will cost too much, and that “containment” will be the better way to go.

The United States and its allies have found no way to counter this strategy. They have imposed trade and financial sanctions, but the sanctions have not stopped the nuclear program. They have warned that military action is possible, but Iran appears not to believe it. They have urged Iran to negotiate, but have lacked the means to extract any sort of concession.

The West is pinning its hopes on negotiations. But this posture misreads the needs of the long game. Iran has no choice but to push ahead. How can it comply, for example, with the U.N.’s demand that it stop enriching uranium? The demand has been repeated in a series of Security Council resolutions. But if Iran agreed, it would be frozen at one bomb’s worth of medium-enriched uranium and five bomb’s worth of low-enriched uranium, not enough to reach an arsenal in a dash, and never enough for a mature program. To make matters worse, Iran would never be able to perfect its new centrifuges. Iran’s nuclear argosy would be stuck on a reef.

This reality explains Iran’s behavior in negotiations. For several years running, it has rejected U.S.-backed offers that would have curbed its capability. And each offer has been more conciliatory than the one before. The most recent, in April, offered to put aside the U.N.’s demands and accept a deal in which Iran would agree only to reduce, not stop, its production of medium-enriched uranium, and to ship part, not all, of its stockpile of that material out of the country. Even that mild proposal was rejected. Iran chose to face more sanctions rather than slow its progress.

Instead the United States is stuck. Iran is not providing an opening—it is not doing anything sufficient to create a crisis. It is tiptoeing forward, building its potential every day, claiming it has a right to enrich uranium (it has no such right, as explained below) and staying in the good graces of its protectors, Russia and China. It is also coping with sanctions and stringing out negotiations. “No breakthrough but also no breakdown,” was what a U.S. official was quoted as saying after the most recent round of talks.

IX. No prevention without red lines

Can such a strategy be countered? Only if the United States actually implements what it says is its policy on Iran. The Obama administration declared last year that it had a policy of “prevention,” that is, a commitment to prevent Iran from getting the bomb. This was different from “containment,” which would try to manage any nuclear threat after it arose. This policy of prevention also had a corollary: to prevent even the “capacity” to build a bomb, which is an earlier phase in development, and different from waiting until Iran is only a screw’s turn away from completion.

There is only one way to apply such a policy: to define the point at which Iran’s ability to make a small arsenal comes into being, and to state what will be done to prevent that point from being reached. In other words, red lines. And statements of what will happen when they are crossed. For example, the United States could declare a limit on the amount of enriched uranium Iran would be allowed to accumulate (the excess would be sent abroad), or declare a limit on the number of centrifuges Iran could deploy (new or old). And the consequences could be a progression. First, more severe sanctions. Then military steps such as interdiction of oil shipments, a blockade of ports, or even air attack. At each step Iran could limit the harm by limiting its program.

But none of this seems to fit the Obama administration’s playbook. Without it, of course, there is no way for the United States to build political support for taking action if Iran is defiant. This last point is vital. If prevention is really the policy, then force has to be an option. But the United States cannot attack Iran out of the blue. The American public would have to be prepared, and so would U.S. allies. The U.S. government would have to remind everyone of the following: that the United Nations has officially condemned Iran for breaching its obligations under the Nuclear Nonproliferation Treaty, that Iran thereby lost any right it may have had under the treaty to enrich uranium, that several U.N. resolutions have declared as much, that the U.N. has called upon Iran to cease enrichment, that Iran has not complied, and that Iran refuses to answer questions from U.N. inspectors about its weaponization work.

To this could be added Iran’s unflagging support for terrorist groups in its neighborhood, its supply of parts for roadside bombs in Iraq that have killed U.S. troops, and its outrageous violations of human rights. All of these things could and would be cited in support of red lines, if prevention were really the policy.

Instead, Uncle Sam has maintained a mild demeanor, intended to nurture negotiations. But they have gone nowhere for years. It is clear why. Iran sees all too acutely that the United States is not ready to set red lines, and is even farther from using force. None of the political ground work has been done, or is likely to be done. Without it, threats aren’t credible. Thus, Iran sees no impediment to its long game. It is working well, and unless something changes, it will give Iran the bomb.

A shorter version of this article appeared as a commentary on Bloomberg View on May 13, 2013. To view this commentary, see "How U.S. Can Break Up Iran's Long Nuclear Game."