"Engaging in a broad-based diplomatic offensive, and beginning a redeployment of U.S. forces in Iraq, represents the best way to secure America's interests in the region and combat the serious threat of terrorist networks," Holbrooke, who served under President Clinton, said in the Democrats' weekly radio address.

Britain will withdraw about 1,600 troops in the coming months and aims to cut more by late summer. The announcement came as Bush is implementing his plan to send an additional 21,500 combat troops to Iraq.

"Like our allies, the Bush White House needs to acknowledge some unavoidable, if unpleasant, facts on the ground. Plain and simple, there are not, and never have been, enough troops in Iraq to accomplish the mission as stated by President Bush," Holbrooke said.

The Bush administration has said Britain's troop cutback shows success in the region.

Holbrooke said withdrawing troops will give the U.S. more flexibility in fighting terrorists and will encourage the Iraqi government to take on more responsibility. He called for increased diplomatic efforts with countries who have a stake in Iraq's future, such as Syria and Iran.

Meanwhile, Senate Democrats are drafting legislation to require the gradual withdrawal of U.S. combat troops beginning within 120 days after the bill's enactment.

“As Americans, we should ask: Why are thousands more of our troops being sent to Iraq just as British troops are planning to leave?" Holbrooke said.