Iraq has become the place where America is meeting the single most important challenge of our time: Islamic extremism. So critical is the encounter that Americans need to snap out of their Iraqi blues and focus on what matters.

Short of securing the place, a walkout now would mean ceding Iraq to a pool of Islamic jihadists, Al Qaeda nihilists, Iranian mullahs, and Syrian muckrakers, in the process abandoning 350 million Arabs to some kingdom of darkness and losing control of the world’s largest energy reserves. That is not even a slight exaggeration.

It would fling doors wide open to a ‘’Reverse Crusades,” past Arab and Muslim gates and into global jihad.

Senator Lieberman of Connecticut correctly assessed the situation in the Wall Street Journal Saturday: “Iraq is the central part of a larger and ultimately longer-term conflict in the Middle East between moderates and extremists, between democrats and dictators. Are we going to surrender to them, surrender that country to them, and encourage people like them to be in authority and power all over the Middle East and in a better position to strike us again?”

The leading actor in all of this is Iran, not America. Obsessing over the Bush administration’s plans to open a second front with Tehran overlooks the fact that Iran’s is a young revolution, less than 30 years of age, representing the cutting edge of an Islamic, albeit Shiite, revival filled with energy, commanded by intelligent and resourceful clergy, funded by oil money, armed to the teeth, and fired up by hubris. Hegemony is built into its DNA.

Iranians already launched a wider conflict last summer via their proxies in Lebanon, the Palestinian Arab territory, and Iraq. They are not about to stop.

True, the U.S. did Iran a big favor by knocking out its arch-enemy, Saddam Hussein, but President Bush is not wrong to tag it as a constant menace. Notwithstanding occasional talk of dissidence — a few demonstrations here and there, and voices from a rich diaspora — a vast majority of Iran’s 72 millions remain firmly aligned with the clergy.

If anything, the mullahs are right to smell American blood now. Engineering setbacks for America in Iraq is a natural follow-up to last summer’s Iranian-inspired triumphs against moderates in Lebanon and Israel.

With America out of the way, Iran and its Islamist Middle East allies have nothing to halt their quest to topple weak, corrupt, ‘’infidel” elites in Egypt, Saudi Arabia, and Jordan, as well as effete princes and rulers of smaller Gulf principalities. Why would Iran refrain from shaking trees laden with ripe fruit?

Well-intentioned observers argue that none of the troubles in the Middle East should be America’s business anyway, and that for all the noise the U.S. survived the loss of Vietnam. In fact, Iraq in many ways is far more important.

In Vietnam, America left with a bruised ego and a dent in its superpower armor. In the Middle East, it would, among other things, be leaving a huge pool of energy, 60% of world reserves, in the hands of an Iranian-dominated region caught up in a frenzy of Islamic militancy — much of which is directed at the West and its economic interests.

This cannot be written off, especially because the balance of world energy is controlled by Russia, a rival.

More significantly, an abrupt departure from Iraq would leave a Muslim universe energized by jihadists and their leaders, who more than ever would be persuaded that they once more triumphed against pagans after defeating the infidel Soviets in Afghanistan in the 20th century. The call for jihad would be resounding around the planet. How many Muslims in good standing — especially those residing within the Western universe — would resist a divine temptation to grab the rest of the globe?

Then there is Israel, a lone Westernized, let alone Jewish, bastion in a region where nearly all around would wish it burned to the ground. After last summer’s war with the Iranian-sponsored Hezbollah and Hamas, the country is once again enveloped in layers of vulnerability to jihadists within and on its borders.

Without America in the neighborhood, attacks would surely resume.

Leaving Vietnam in defeat was bitter. Leaving Iraq without settling it would be the start of a bigger catastrophe.