Addressing pilgrims on November 5, 2011, just over two weeks after President Obama announced U.S. troops would withdrawal from Iraq, Iran's Supreme Leader cited American “failures” in Iraq and Afghanistan as proof that, “Today, the West, the United States and Zionism are weaker than ever before.”

Iranian President Mahmoud Ahmadinejad went further, declaring the American retreat was not enough. “So long as the American empire based in the White House has not been overthrown, we have work to do,” he thundered.

In the weeks since, the Islamic Republic has ratcheted up both its rhetoric and its defiance.

Whereas Iranian authorities often harass Americans visiting family in Iran, its January 8 sentencing to death of Amir Mirzaei Hekmati, a former U.S. Marine in the country to visit his grandparents, appears resolute. Hekmati is a now pawn to an Iranian desire to prove American impotence.

Iranian defiance has been particularly shrill on the nuclear issue. Tehran dismissed as illegitimate the most recent International Atomic Energy Agency (IAEA) report which listed a litany of Iranian activities that appear geared more to a weapons program and less to energy production. In the wake of the report, even Chinese and Russian diplomats find it difficult to maintain the fiction that Iran’s nuclear program is motivated solely by energy needs. Generating electricity to power factories does not require designing bomb triggers.

Tensions—and a sense that time is running out to stop Iran’s nuclear ambitions—increased with Sunday’s announcement that Iran had commenced uranium enrichment in a once-secret underground facility which the Islamic Republic acknowledged only after confronted by the IAEA. Meanwhile, Ahmadinejad, in Latin America visiting anti-American leaders, ridiculed American concerns.

Iranian antagonism toward the United States took an even more dangerous turn in the Persian Gulf.

On December 28, 2011, Iranian authorities threatened to close the Strait of Hormuz, the 34-mile wide waterway through which more than one-third of the world’s oil tanker traffic passes. Habibollah Sayyari, commander of Iran’s navy, likened the ease of closing the Strait to “drinking a glass of water." In recent days, Iranian officials warned the USS John C. Stennis, an aircraft carrier currently in the region, not to re-enter the Persian Gulf after it transited the Strait during an Iranian exercise.

While Iranian authorities have threatened traffic in the Strait before, they have not done so in almost a quarter century, since President Ronald Reagan responded with force—through Operation Praying Mantis—to defend international waters in the Persian Gulf, handing the Iranian navy the worst defeat in its history.

That Iranian officials would once against threaten a U.S. warship suggests that Tehran no longer believes the United States has the will or ability to defend its interests.

In the wake of Operation Praying Mantis, Iranian authorities came to respect Reagan, even as they hated him. Today, with his unilateral retreat from Iraq, Iranians of all stripes realize that Obama is no Reagan.

Time is running out. Analysts today may dismiss Iranian threats as bluster, secure in the knowledge that Iran has yet to develop nuclear weapons. But, Tehran’s behavior should indicate the true consequences of an Iranian nuclear breakout. As soon as the Islamic Republic acquires nuclear weapons capability, its bluster will become reality.

Secure behind its own nuclear deterrent, Iranian authorities may pursue their ideological goals with impunity. It is one thing for an American aircraft carrier to defy Iranian demands and face down lightly-armed speedboats, but is quite another to risk a confrontation with a trigger-happy nuclear power.

Not only could Iranian authorities hold the international economy hostage—the net effect of acquiescing to Iranian claims to be the gatekeeper over the Strait—but they also could threaten America’s strategic presence. In recent years, Iranian officials have revived a centuries-old territorial claim to the island nation of Bahrain, home to America’s 5th Fleet, utilizing much the same rhetoric which Iraqi dictator Saddam Hussein once used to describe Kuwait.

When April Glaspie, the American ambassador in Iraq, then signaled an unwillingness to defend Kuwait, Saddam pounced. If Iranian authorities believe the United States too weak to respond, Bahrain could face the same challenge from the other side of the Persian Gulf.

Ahmadinejad’s travels through Latin America should also raise a red flag in American national security circles. When the Obama administration proposed setting up a red phone hotline to lessen the chance for conflict spinning out-of-control in the Persian Gulf, Ali Fadavi, the Islamic Revolutionary Guard Corps naval chief dismissed the gesture, declaring that the best way to avoid conflict in the Persian Gulf would be for the United States to evacuate all its forces. He added, however, that once Iranian ships sailed into the Gulf of Mexico, Iranian authorities would consider a red phone hotline to U.S. forces in that region.

Diplomats may dismiss Hugo Chavez as a tragic buffoon who guided once wealthy Venezuela into poverty, but if Iranian rhetoric is to be believed, a replay of the Cuban missile crisis could be around the corner, a half century after President Kennedy stared down Soviet counterpart Nikita Khrushchev.

Iranian bluster is bad enough. When Tehran is able to put substance behind it, American interests will truly be in peril. The question for Obama and the Republicans seeking to replace him is whether the United States can bear an Iranian challenge which will grow exponentially once Iran goes nuclear.

Michael Rubin, a resident scholar at the American Enterprise Institute, is a senior lecturer at the Naval Postgraduate School.