The United States has concluded a major phase of military withdrawal seven years after it led the invasion of Iraq.

The last designated U.S. combat brigade left Iraq in the early hours on Thursday and has entered Kuwait. However, 50,000 American troops would remain stationed in Iraq till end of 2011. They have been labelled as advisers and trainers for the Iraqi forces.

Analysts, however, point out that several factors including the internal situation prevailing by the end of next year is likely to influence further withdrawals. At their peak, nearly 150,000 U.S. troops had been stationed in Iraq.

Despite Thursday's pull-back, the remaining U.S. troops in Iraq have significant combat capability. They include Special Forces, as well as others, including helicopter gunship crews, who have substantial air power at their command. The U.S. Air Force is expected to operate in Iraq till around 2018, to make up for the existing absence of a credible Iraqi air force.

Observers say the final withdrawal is uncertain, as Washington may find it difficult to quit entirely, so long as there is a threat from Iran, which could fill the political space that is likely to emerge once the U.S. forces withdraw completely. The contours of U.S.-Iran relations in the coming months are therefore likely to be the key factor influencing Washington's final withdrawal time-table.

According to a U.S. military spokesman, armoured vehicles began ferrying into Kuwait, the fourth Stryker Brigade, second infantry division, in the early hours of Thursday. “The last one crossed at around 0600 this morning,” Lieutenant Colonel Eric Bloom was quoted as saying.

U.S. State Department spokesman P.J. Crowley said Washington's engagement in Iraq was far from over. However, U.S. role from now on would be more civilian focused.

More than 4,000 U.S. troops have been killed in Iraq since the U.S.-led invasion began in March 2003. Civilian death count has ranged between 97,196 and 106,071, according to Iraq Body Count, an independent monitoring group.

There has also been a large exodus of skilled manpower from Iraq, and the Iraqi middle class has been virtually wiped out, in the aftermath of the war, significantly hampering reconstruction work, analysts say.

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