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Editor's note: Vali Nasr is a professor of international politics at the Fletcher School of Law and Diplomacy of Tufts University and senior fellow in foreign policy at the Brookings Institution.

(CNN) -- Iran has threatened that it will retaliate against the Obama administration's proposed new economic sanctions on Iran's oil exports by blocking the flow of oil from the Persian Gulf. "If sanctions are adopted against Iranian oil," said Iran's Vice President Mohammad Reza Rahimi, "not a drop of oil will pass through the Strait of Hormuz," the narrow waterway at the mouth of the Persian Gulf, which one-fifth of the world's oil supply passes through daily.

To drive the point home, Iran has started a 10-day naval exercise in the Persian Gulf to show off how it could use small speedboats and a barrage of missiles to combat America's naval armada. And the U.S. Navy has responded, in the words of a spokeswoman: "Anyone who threatens to disrupt freedom of navigation in an international strait is clearly outside the community of nations; any disruption will not be tolerated."