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The secretary said he has served every president since Lyndon Johnson, and that in his career, Iran has proved "the most frustrating and challenging national security problems the United States has faced." If Iran acquires nuclear weapons, he said, an arms race will ensue in the region.

"To have a proliferation problem in the most volatile part of the world cannot be a good development," he said. "On the other hand, I think - as we've seen in Iraq - that every war is unpredictable and has unintended consequences and is more difficult than people expect. I think a military attack upon Iran would have enormous consequences in a variety of ways."

Still, an attack remains an arrow in the quiver, the secretary said, and Obama needs all options open.

"All the evidence … shows the new round of sanctions is really beginning to bite on the Iranians," the secretary told the audience. Iranian leaders hate to be isolated, he said, and they are being isolated. Now, he added, countries in the region and elsewhere are tightening the sanctions on Iran, and that could have an effect.

The only long-range solution to the problem is for the Iranian government to decide that its nuclear program diminishes their country's security rather than enhancing it, Gates said. In the meantime, he added, the United States is working with allies in the area to provide missile defense and to keep the diplomatic front open.

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