As President Obama walked from the podium after announcing his plan to withdraw all troops from Iraq, a White House reporter shouted out, “What about Iran?”

Obama ignored the question because he could, but, sooner or later, he will have to deal with Iran. Make that sooner.

The mere reminder of the Greatest Threat on Earth brought a sobering end to Obama’s victory speech. Beyond the surprising withdrawal from Iraq, which he called proof of “success,” he boasted of winding down the war in Afghanistan and the deaths of Muammar Qaddafi and Usama bin Laden.

Those things happened on his watch, and the president deserves credit, though his overuse of his favorite personal pronoun -- I -- obscures the basic continuity with the Bush administration’s war on terror.

Yet no matter the author, the winning streak is not writ in stone as long as Iran is free to stir the pot. If the “Arab Spring” becomes the “Arab Winter,” it will largely be because Iran sees instability as its friend and, absent U.S. troops, there will be no force to stop it.

Iran is the world’s biggest troublemaker, and the clock is ticking on its quest for nuclear weapons. Even without that terrifying prospect, it sponsors terror groups such as Hamas in Gaza, Hezbollah in Lebanon, and trains and arms the militias killing our soldiers in Iraq.

It supports the Taliban in Afghanistan, and, last summer, the Obama administration linked Iranian leaders to Al Qaeda. It foments violence in Yemen and Bahrain. Saudi Arabia, the base of Sunni power, is engaged in proxy skirmishes with Iran throughout the region.

The alleged plot by Iranian agents to kill the Saudi ambassador in Washington, D.C. shows Iranian aggression has reached our shore, and that is not the only example. It developed strong ties to Hugo Chavez in Venezuela to export trouble closer to us.

Israel, despite a long cold war with Syria, is worried about what will happen if the brutal Assad regime falls. When I asked an Israeli official why he wasn’t cheering the chance Assad might collapse, he gave a quick answer: “Iran.” It could become like Lebanon, another Iranian satellite state.

Which brings us back to Iraq.

One of the unintended consequences of the American-led invasion in 2003 was that it removed the best buffer to Iranian expansion. Saddam Hussein had fought a long war with Iran and served to block the export of the Shiite Islamic revolution that started in 1979.

Without Saddam, the gates to the Arab world were opened. And without American troops in Iraq, its dominant Shiite population could easily fall into the Iranian sphere.

That is the concern of Sen. John McCain and others who believe Obama is risking the stability America won at such a high price by pulling out all the troops now.

Indeed, many of Iraq’s power players are closer to Iran than to the United States. Prime Minister Nouri al-Maliki spent years in Iran hiding from Saddam and visited venomous President Mahmoud Ahmadinejad during his 2010 re-election campaign.
“The strategic relations between Iran and Iraq must continue,” Maliki said then.

Obama faces a re-election fight of his own and is eager to claim that, as promised, he ended the unpopular wars. If our troops do come home before Christmas, he will get a political boost.

But the troop withdrawal is not the end of the story.

Iran will try to fill the vacuum and turn Iraq into its cat’s-paw. That would present new dangers to our national security that no president can ignore.

Ultimately, Obama or his successor must answer the question he ducked last week. Until Iran is defanged, the Mideast will remain unstable and peace will remain elusive.

Michael Goodwin is a New York Post columnist and Fox News contributor. To continue reading his column on other topics, including Herman Cain and Martin Luther King's dream for America, click here.