WASHINGTON (Reuters) - Less than two weeks after the fighting ended, the United States is coming face to face with a huge dilemma surrounding its efforts to rebuild Iraq.

Simply put it is this: the longer U.S. occupation forces stay in Iraq, the greater the risk of fueling anti-American Islamic fundamentalism in the country.

But the sooner they depart, the more of a mess they will leave behind, which could have the same result, as well as creating a power vacuum that anti-American forces could fill.

"The longer we stay, the more Iraqi nationalism will get organized and become a unifying force, which will be expressed in increasingly strident opposition to the American presence," said Anne-Marie Slaughter, dean of Princeton University's Woodrow Wilson School of Public and International Affairs.

"But if we leave prematurely, the risk exists that a Shi'ite-led clerical government will emerge that is allied to Iran and professes an anti-American theocracy," she said.

SURPRISED BY SHI'ITES

U.S. officials have been surprised by the speed with which Shi'ite Muslims, who comprise 60 percent of the Iraqi population but have never led the nation, have asserted themselves in the vacuum left by the fall of Saddam Hussein.

"I'm sure the Bush administration is shocked by the emergence of the Shi'ites but now they have unleashed something and I don't know what they can do to stop it," said J. Brian Atwood, a former head of the U.S. Agency for International Development, now dean of the Humphrey Institute of Public Affairs at the University of Minnesota.

"Having a U.S. administrator wandering around Iraq talking about introducing democracy as if he's selling an alien ideology is beside the point," Atwood said.

The long-standing links between the Shi'ite clerics emerging in leadership roles in southern Iraq and their fellow Shi'ites in Iran threatens to shorten whatever honeymoon U.S. troops can expect, according to Joseph Braude, author of the recent book, "The New Iraq: Rebuilding the Country for Its People, the Middle East, and the World."

"It's clear that Iran is already exerting its influence and it seems the Iranian government would like to see a government of clerics emerge in Iraq that is similar to itself," Braude said.

That outcome would be strategically unacceptable to Washington, which will need to find and foster moderate Iraqi political forces to prevent it. So far, it is not really clear what the majority of the Iraqi people want.

Some conservatives in the Pentagon and elsewhere in the U.S. government had apparently expected Iraqi exile leaders like Ahmad Chalabi to return to Iraq and fill the leadership vacuum. But Chalabi has failed to ignite much enthusiasm among fellow secular Shi'ites and totally lacks appeal to religious Shi'ites.

"Overall, the administration has put stunningly little planning into how the political process might unfold in Iraq after the war," said Nancy Soderberg who served on the National Security Council under former President Bill Clinton and now works with the International Crisis Group in New York.

"Their ideological assumptions have constantly been confounded by reality," Soderberg said.

Braude said there was no question of U.S. forces withdrawing from Iraq at least until it was sure it was leaving a viable state that had the ability to maintain its territorial integrity.

That could take years since it means building a new, ideologically revamped Iraqi army of perhaps 150,000 -- the minimum sufficient to defend against an Iranian military next door of half a million. The two countries were at war for much of the 1980s.

In the meantime, the Americans and their Iraqi partners face the monumental task of rebuilding a legal framework that could help run a society based on the rule of law in which commercial life can flourish free of corruption.

That means writing criminal and civil laws, recruiting police, judges, lawyers, prison administrators and eventually writing a constitution. At this point, no one knows who will do these things and when.

Many analysts, including Slaughter and Soderberg, believe that President Bush will have no choice but to swallow his reluctance to invite the United Nations to help share the task of running Iraq until sufficient physical, political and legal infrastructure is up and running.

Democratic Sen. Joseph Biden has been making this point for weeks, even before the war began March 19. "Acting under a U.N. flag, as opposed to a U.S. flag, will minimize resentment from malcontents in the region and beyond," Biden said March 11 during a Senate Foreign Relations Committee hearing on humanitarian assistance and reconstruction in Iraq.

"The benefits of working with the international community cannot be overstated," the Delaware lawmaker said.

But Atwood said Bush seemed to have an ideological blind spot when it came to the United Nations. "They need to get over it and get the U.N. in there now," he said.