CAIRO (AP) ― The U.S. military’s departure from Iraq opens the door to expanded Iranian influence in the Middle East, though that door could close fast if Iran’s closest Arab ally Bashar Assad falls from power in Syria.

That’s among the uncertainties looming over the Middle East in the wake of President Barack Obama’s decision to remove all U.S. troops by the end of this month, fulfilling a campaign promise to end the unpopular war and abandoning efforts to negotiate an extension of the year-end deadline agreed to by the Bush administration in 2008.

At first glance, that would make Iran the big winner, especially if the U.S. move heralds a tectonic shift of power in the strategic Persian Gulf region as the United States shifts its military focus to East Asia and the Pacific. But the tumult from the Arab Spring, on top of the end of the nearly nine-year Iraq War, has made the rivalry between Iran and the U.S.’s Arab allies even trickier and predictions more cloudy.

No longer will tens of thousands of American troops be stationed along Iran’s western border. They are leaving behind an Iraqi government dominated by Shiite Muslim parties beholden to the Iranians, who sheltered them for years when Saddam Hussein and his Sunni-dominated Baath regime were in power.

With the American military presence reduced to a few hundred members of an embassy-based liaison mission, Iran is likely to step up infiltration of Iraq’s intelligence services ― the key to manipulating Iraq’s internal politics ― and expand its links to both Shiite and Kurdish politicians, to the alarm of the country’s Sunni minority.

As the second most populous country in the Gulf, with some of the world’s largest proven petroleum reserves, an avowedly pro-Iranian Iraq would be a game changer in the power struggle between Iran and the U.S.-backed, conservative Gulf states led by Saudi Arabia.

Iran already wielded considerable influence in Iraq even when U.S. troop strength approached 170,000. The U.S.-led invasion of 2003 produced a strange alliance between the Americans and religiously based Shiite parties tied simultaneously to both Washington and Tehran. Prime Minister Nouri al-Maliki, a Shiite who had been cool toward Iran, has moved closer to the pro-Iranian groups since a political crisis in 2010 nearly cost him his job.

With the American military gone, Tehran’s prospects for bolstering those ties in Iraq look bright.

At closer examination, however, the future appears less certain. Much will depend on how the key players ― including the United States ― maneuver diplomatically through the new environment created by the end of the Iraq War.

“The United States must succeed in limiting and countering Iranian influence in Iraq and in creating Iraqi forces that can defend the country,’’ wrote analyst Anthony Cordesman of the Center for International and Strategic Studies. “The United States must also restructure a mix of forward-deployed U.S. forces and ties to regional powers that can contain every aspect of Iran’s military forces and political ambitions.”

Iran’s ability to manipulate a post-America Iraq is by no means unlimited, in part due to a flowering of Iraqi nationalism which survived the horrific bloodshed of the Sunni-Shiite sectarian war.

Many ordinary Iraqi Shiites harbor bitter memories of the 1980-1988 war with Iran, when young Shiite soldiers bore the brunt of the casualties. Among the Sunni minority, hostility to Iran runs even deeper, and much of the talk of Iranian domination stems from overblown comments by Sunni politicians seeking to discredit their Shiite political rivals.

“The Iraqis have no desire to be a client state of their Persian neighbor,’’ said Doug Ollivant, senior fellow at the New America Foundation and former director for Iraq on the National Security Council in both the Bush and Obama administrations. “This is a real threat ... but the threat is overstated.’’

An Iranian strategic victory in Iraq could also be checkmated by a regime change in Syria, where Assad is facing mounting resistance to his family’s autocratic 40-year rule.

The fall of the Assad regime would be the biggest blow to Iranian foreign policy since the Iran-Iraq war and would render Tehran’s international isolation nearly complete.

In strategic terms, Syria is an even bigger prize for Iran.

Syria is Iran’s bridge into Lebanon, a near-client state of Damascus where the militant Shiite movement Hezbollah has flourished for decades with Iranian support, channeled through Syria. Iran would also lose its conduit to the radical Palestinian group Hamas, which is headquartered in Damascus.

With the stakes high and so much still shrouded in uncertainty, the major players in the region are scurrying to retool their political strategies.

Fearing that Iraq will eventually fall into Tehran’s orbit, Saudi Arabia and its Gulf partners have focused on Syria in hopes of containing Iran’s influence. The key to that is to cut the links between Iran and Syria, and to accomplish that, the Gulf states have been at the forefront of moves by the Arab League to impose sanctions against Syria over the killing of protesters.

At the same time, the Saudis are pushing their Gulf partners into bolstering support for Jordan, which also borders Iraq, Egypt and Morocco in hopes of maintaining a strong bloc of Sunni Arab states ― some of whom have been shaken by the Arab Spring ― in the face of a resurgent Iran.

In the end, all these scenarios will be influenced by what happens in Iraq now that the American troops have gone.

“Tehran will likely react to that power shift by trying to build more influence in Iraq,” wrote analyst Paul Salem of the Carnegie Endowment for International Peace. “However, Iranian influence over tomorrow’s Baghdad is not a foregone conclusion. Iraqi and Arab nationalism run deep in Iraq, and Iran might find it as hard to dominate Iraq as the Americans did.’’