While the last brigade of American combat troops began to leave Iraq Thursday, the Obama administration planned to more than double the number of private security guards it has in the country to fill the void.

About 50,000 noncombat U.S. troops are expected to remain to advise and assist Iraqi forces. The combat forces' exit comes ahead of the Aug. 31 deadline set by President Obama.

When the last U.S. troops leave at the end of 2011, the Obama administration plans to more than double the number of private security guards it has in Iraq -- up to 7,000 -- The New York Times reported Wednesday, citing unnamed administration officials.

The State Department move is aimed at protecting civilians still exposed to Al Qaeda-linked insurgents and Iranian-backed militias.

Contractors employed by the State Department will train the Iraqi police and U.S. diplomats in two new $100 million outposts will be left to defuse sectarian tensions in northern Iraq.

The security contractors, defending five fortified compounds around the country, will operate radars to warn of enemy rocket attacks, search for roadside bombs, fly reconnaissance drones and staff quick reaction forces to help civilians in distress, the officials told the Times.

As the exit of combat troops was shown live on TV in the U.S. Wednesday, U.S. State Department spokesman Philip Crowley described the end of combat operations as a "historic moment," but vowed that America's long-term commitment was unwavering.

Pentagon spokesman Bryan Whitman said combat operations will officially end Aug. 31, and on that date the mission will change from Operation Iraqi Freedom, to Operation New Dawn.

The troops of the 4th Stryker Brigade, 2nd Infantry Division, after passing into Kuwait, will then return to the United States over the next few days.

As the 4th Stryker Brigade's convoy reached the barbed wire at the border crossing out of Iraq on Wednesday, the soldiers whooped and cheered. Then they scrambled out of their stifling hot armored vehicles, unfurled an American flag and posed for group photos.

Before the Aug. 31 deadline, about half the brigade's 4,000 soldiers flew out like most of the others leaving Iraq, but its leadership volunteered to have the remainder depart overland. That decision allowed the unit to keep 360 Strykers in the country for an extra three weeks.

U.S. commanders say it was the brigade's idea, not an order from on high. The intent was to keep additional firepower handy through the "period of angst" that followed Iraq's inconclusive March 7 election, said brigade chief, Col. John Norris.

It took months of preparation to move the troops and armor across more than 300 miles of desert highway through potentially hostile territory.

The Strykers left the Baghdad area in separate convoys over a four-day period, traveling at night because the U.S.-Iraq security pact -- and security worries -- limit troop movements by day.

Fox News' Mike Emanuel and Justin Fishel and the Associated Press contributed to this report.