Operation Desert Storm,

Twenty years ago, Saddam Hussein's forces invaded Kuwait, setting in motion US military action that is only now coming to a close, as American troops prepare to withdraw from Iraq.

The ill-advised move by Saddam triggered a US-led response that quickly pushed his army out of Kuwait, followed by years of military tension culminating in the American invasion of Iraq seven years ago.

The 1990-1991 Gulf war, dubbed "Operation Desert Storm," enjoyed wide international support, even among Arab countries, while the 2003 invasion stirred controversy and bitter opposition around the world.

As Saddam's regime was left intact, the first conflict helped plant the seeds of the second, with far-reaching consequences for the balance of power in the Middle East.

As the United States gradually withdraws from Iraq over the next year under a security pact, the US legacy in Iraq remains the subject of intense debate. Critics question if it was worth the terrible cost in human life and the damage done to Washington's image abroad.

"On balance the costs of our policy have been very high -- higher than need be, perhaps higher than the benefits will warrant," said Michael O'Hanlon, a senior fellow at the Brookings Institution think-tank.

"But there are indeed huge benefits and huge potential benefits," O'Hanlon said.

With Saddam's regime toppled, the United States can look to Iraq as an ally, albeit still plagued by serious ethnic and sectarian divisions. Moreover, Washington no longer has to worry about a dangerous dictator in the region hostile to America and its allies.

But former president George W. Bush's vision of a new democratic Iraq transforming the Middle East looks unrealistic and naive, said Stephen Biddle of the Council on Foreign Relations.

The end of Saddam's dictatorship lifted the lid on a Pandora's box of tensions between Kurds and Arabs and Sunni and Shiite communities, he said.

"I wonder whether the demonstration effect would be the opposite of what we hoped it would be," said Biddle.

The best the United States can hope for is an Iraq that is "imperfect but stable," he said.

In another unintended consequence, the removal of Saddam bolstered Washington's arch-foe Iran, with Tehran forging links with leading Shiite parties amid a political vacuum.

For the American military, Iraq under Saddam preoccupied the top brass for two decades.

The decisive victory in the first Gulf war came as a vindication, burying the ghosts of the Vietnam War as hi-tech weaponry quickly rolled over Iraqi forces.

The Gulf war represented "the phoenix rising from the ashes of Vietnam," said David Johnson, a retired colonel and an analyst at the Rand Corporation.

Although America's public faith in the US military was restored, success in 1991 may have contributed to the Bush administration's over-confidence before the 2003 invasion, with little thought given to what would follow the fall of Saddam's regime.

As sectarian violence spiralled out of control in the years after the invasion, Bush sent in yet more US forces as commanders embraced a radically different approach, drawing on counter-insurgency doctrine that had been discarded after Vietnam.

"It took a superpower within a millimeter of its capacity to get it under control," Biddle said.

Fighting the war in Iraq required repeated combat tours for the all-volunteer US force, putting unprecedented strain on the armed forces.

The war also drained resources from the fight against the Taliban in Afghanistan, and as a candidate, Barack Obama called the Iraq conflict a dangerous distraction from the fight against Al-Qaeda and its associates.

As US president, Obama inherited a security agreement with Baghdad that calls for all US forces to pull out by the end of 2011. He has ordered the force to draw down to 50,000 by September 1.

It is too early to gauge the effect of the Iraq wars, though most analysts judge the 2003 invasion as an unnecessary disaster that permanently dented US influence, particularly among Islamic countries.

Some experts, however, say any assessment has to take into account what might have evolved without the US intervention.

According to Biddle, Saddam likely would have pursued nuclear weapons with relentless determination.

"There is some reason to speculate that he would eventually have been in the same position that Iran is in now," suspected of being poised to secure atomic weapons, he said.