Iranian President Mahmoud Ahmadinejad. Photo: www.kremlin.ru.

Since the “Arab Spring” began four months ago in Tunisia, U.S. media have focused constantly and generally optimistically on the turmoil in the Middle East. Unfortunately, the rising threat of an Iranian Winter—nuclear or otherwise—is likely to outlast and overshadow any Arab Spring.

Iran’s hegemonic ambitions are embodied in its rapidly progressing nuclear-weapons program and its continued subversion across the region. In a case that emphasizes the fragility of aspiring democracies, Iranian Winter has already descended upon Lebanon, where Iran’s influence has helped replace a pro-Western government with a coalition dominated by Tehran’s allies, including Hezbollah. Last week, departing Lebanese Prime Minister Saad Hariri condemned Iran’s “flagrant intervention” in his country.

In Syria, despite substantial opposition to the Assad dictatorship, regime change is highly unlikely. Iran will not easily allow its quasi-satellite to be pried from its grasp, and is reportedly helping the Assad regime quell this week’s protests.

Then there’s the Victoria, a ship containing tons of weaponry bound for Hamas that the Israeli navy seized last month. The episode recalls the Karine A, a weapons shipment from Iran to the Palestine Liberation Organization seized by Israel in 2002. Clearly Iran has a penchant for arming Sunni and Shiite terrorists alike.

Iran’s support for Hamas is even more important now that Egypt’s blockade of the Gaza Strip, porous as it sometimes was, has now effectively ended with the fall of Hosni Mubarak. Hamas, a wholly owned subsidiary of the Muslim Brotherhood, is now free to transfer arms and operatives between Gaza and Egypt, sowing trouble in both places.

The real tip of the spear in the Middle East may be Bahrain, home of the U.S. Fifth Fleet. No longer oil-rich, the tiny monarchy is separated from Saudi Arabia only by a causeway. Popular protests in Bahrain, a Sunni monarchy ruling a 70% Shiite population, pose the starkest potential conflict between U.S. principles and strategic interests. Iran would happily welcome a “free” election in Bahrain, which would likely bring to power a pro-Tehran leadership.

Troops from nearby Arab states, led by the Saudis, have assisted Bahrain in harshly suppressing Shiite protests. These pro-U.S. monarchies, shaken by Washington’s casual dismissal of longtime ally Hosni Mubarak, also worry about President Obama’s withdrawal from Iraq and planned reductions in forces in Afghanistan. And there is no evidence that the president has a strategy to deal with Iran’s continuing threat in the Gulf region—which extends to Yemen, where Iran could use Shiite tribes to gain influence, even as al Qaeda in the Arabian Peninsula also strengthens.

Inside Iran, we now have confirmation—thanks to disclosures this month by an Iranian opposition group, which have been confirmed by Iranian officials—that the regime has the capability to mass-produce critical components for centrifuges used to enrich uranium to weapons-grade levels. That news proves again the inefficacy of U.N. Security Council resolutions and sanctions against a determined adversary.

Thus Iran’s weapons program proceeds full steam ahead, which only emphasizes to would-be proliferators that persistence pays. Moammar Gadhafi surrendered his nuclear weapons program in 2003-04 because he feared becoming the next Saddam Hussein, but he is now undoubtedly cursing his timidity. Had he made seven years of progress toward deliverable nuclear weapons, there would surely be no NATO bombing of his military today.

An Iranian nuclear capability would undoubtedly cause Saudi Arabia, Egypt, Turkey and perhaps others to seek their own deliverable nuclear weapons. We would therefore see a region substantially more in Iran’s thrall and far more unstable and dangerous for Washington and its allies.

Moreover, America’s failure to stop Iran’s nuclear ambitions—which is certainly how it would be perceived worldwide—would be a substantial blow to U.S. influence in general. Terrorists and their state sponsors would see Iran’s unchallenged role as terrorism’s leading state sponsor and central banker, and would wonder what they have to lose.

The Arab Spring may be fascinating, and may or may not endure. Sadly, Iran’s hegemonic threat looks far more sustainable.

Mr. Bolton, a senior fellow at the American Enterprise Institute, is the author of “Surrender Is Not an Option: Defending America at the United Nations” (Simon & Schuster, 2007).