Now that Iraq has shunned a post-withdrawal U.S. military presence, U.S. President Barack Obama's Plan B for a beefed-up American force elsewhere in the Persian Gulf underlined the extent to which Iran is coming out on top in the region.

"There is no disguising Washington's anxiety about the future direction of events both in Iraq and in neighboring Syria," observed international affairs analyst Simon Tisdall of Britain's Guardian newspaper.

"Nor is it possible any longer to avoid the conclusion that while the U.S. fought the Iraq war, it was Iran that won it."

The arrest of 600 high-ranking figures in Iraq's minority Sunni community by Iraqi Prime Minister Nouri al-Maliki's Shiite regime indicates that Tehran wants to eliminate its longtime adversaries.

The roundup by overwhelmingly Shiite security forces, built up over the last couple of years by Maliki, once one of Saddam's most-hunted enemies, has been attributed to a Libyan tip-off that Sunni Baathists were plotting a coup in Baghdad.

Whether there actually was a plot may never be known. Certainly its prospects of success, even if clandestinely supported by the Saudis and their Sunni allies, couldn't have been high.

Whatever, it gave Maliki, who lived in exile in Iran for 20 years before returning home after Saddam was toppled, the pretext to get rid of his political rivals who had been protected by U.S. forces.

The arrests also underlined the dictatorial instincts he has displayed, in the manner of Arab strongmen, since he came to power in May 2006.

Now in his second term, he insists on holding the key ministries of defense, interior and national security for himself. These give him total control over security forces.

The Americans, as well as Iraq's fearful Sunnis, consider him far too close to Tehran for comfort.

Maliki, like the Iranian regime, supports maintaining Bashar al-Assad's widely discredited, Alawite-minority regime in power in Damascus while keeping Syria's majority Sunnis subjugated.

If the beleaguered Assad, propped up by these allies, can crush a 10-month-old popular insurrection against his 41-year-old regime, Tehran will control Iraq, Syria and Lebanon, a dagger aimed at Israel and the Suez Canal.

This is anathema to the Americans and the Saudi-led Sunnis across the region.

The roundup could conceivably be linked to the recent eruption of separatist thought in the largely Sunni province of Salahuddin, which possibly could be attributed to the U.S. withdrawal.

Like most Iraqi Sunnis, those in Salahuddin are fearful of what lies ahead now that the Americans aren't around to guarantee their safety.

In another Sunni-dominated province, Anbar, long a hornet's nest of Sunni insurgency, a local council in Tikrit, Saddam's hometown and a Baathist bastion, began the process of creating an autonomous region similar to the Kurdish enclave across three northern provinces.

The war-hardened Kurds, Saddam's nemesis, are likely to get more independence-minded, signaling serious trouble for Maliki.

There have even been rumblings of separatist discontent too in the Shiite south, where two-thirds of Iraq's known oil reserves of 143 billion barrels lie.

This is largely because the province of Basra sees little of the vast oil revenues that go to Baghdad and because of lingering intra-Shiite differences that go back decades.

But taken together, all these factors must confront Maliki with an uncomfortable prospect and the definite possibility of renewed sectarian bloodletting -- something both Shiite and Sunni armed groups seem bent on triggering.

Might ethnically diffuse Iraq actually disintegrate in the post-American era? What would Iran do? What would the Saudis do, with the way open to Iranian invasion?

From the U.S. standpoint, further upheaval in Iraq, whether instigated by Iran or not, can only mean instability in the Persian Gulf.

The Saudis and the other gulf Arab monarchies are deeply concerned at the way Obama abandoned Hosni Mubarak in Egypt in February, the Americans' waning power in the region and even at the coming withdrawal from Afghanistan.

It's not clear what size of military force Obama plans to deploy in the gulf but it will likely rely heavily on U.S. air power and the U.S. Navy 5th Fleet, which has headquarters in Bahrain.

For the Arabs, the U.S. pullout from Iraq is an Iranian triumph that has left a dangerous vacuum.

Plan B doesn't look like it's going to calm Sunni fears, anywhere.