WASHINGTON: A massive military rotation now under way in Iraq calls for more than 200,000 US troops to trade places as fresh divisions of soldiers and Marines begin a second year of American presence in the fractious nation.

Already, senior military leaders are worried that the sheer scope of the move, unprecedented since World War II, will leave US troops crisscrossing Iraq more vulnerable to attack and increase bloodshed. Outside Iraq, the supply of combat-ready troops available for a major conflict elsewhere will dip.

The equivalent of 8 1/2 of the army's 10 active-duty divisions will be on the move through May. The four army divisions coming home after yearlong deployments are expected to fall below full combat-ready status due to troop leave and equipment repairs, military sources said.

"It is a moment of pretty amazing vulnerability that'll last for a couple months," said Thomas Donnelly, a defence analyst with the American Enterprise Institute, a conservative think tank here.

In addition, nearly half the new troops going into Iraq are reservists - up from 20 per cent now - prompting worries about a fall-off in expertize and experience at the very moment when commanders expect insurgents to test the new arrivals.

"We've got to do this thing right," said a senior army officer who briefed reporters on the rotation. "It is still a dangerous place." The shift of US forces, which begins in earnest this spring, will take place against the backdrop of political turmoil inside Iraq, as the US-led provisional authority prepares to hand over control to an interim government by July 1. That, too, is expected to add to the instability as rival ethnic groups vie for power, and as former Baathists once loyal to Saddam Hussein seek to strike an early blow hoping to derail the new government.

"There will be some natural degradation of capability as you take out seasoned, experienced troops who built up personal relationships and had their instincts honed by months of duty, and you put in brand-new troops. The question is ... how fast do you get up to speed at this critical moment in the political process?" said Sen. Jack Reed, D-R.I., a member of the Senate Armed Services Committee.

"This might be one of their (the insurgency's) last shots, because if we can successfully make this transition, it's a huge step forward in terms of the stabilization plan, so you can anticipate a lot of activity against this whole process, and this complicates the rotation," Reed said.

The rotation grew in size after US pleas for a third international division in Iraq fell flat. Also, the Pentagon was reluctant to extend deployments in Iraq beyond one year, so the switching of several units converged. Most of the 123,000 troops in Iraq now are leaving, and about 115,000 will take their place.

Four army divisions are leaving Iraq - the 4th Infantry, the 1st Armoured, the 82nd Airborne and the 101st Airborne. They will bring with them 650 helicopters, 5,700 tanks and other tracked vehicles and 46,000 wheeled vehicles, an army official told reporters.

The four divisions are being replaced by two army divisions, the 1st Infantry and the 1st Cavalry, and the 1st Marine Division, along with three Army National Guard brigades and an active-duty army brigade using the new Stryker armoured infantry carrier.

The army also is replacing about 20,000 troops in Kuwait, and 10,000 in Afghanistan, where the upstate New York-based 10th Mountain Division is being replaced by part of the 25th Infantry Division.-Dawn/The LAT-WP News Service (c) Newsday.