WASHINGTON - The U.S. military in Iraq is undergoing its biggest changeover in senior commanders since Gen. David Petraeus launched a new counterinsurgency strategy nearly a year ago.

The high-level shifts come at a particularly delicate stage in the war as U.S. troop levels begin to decline, Iraqis are handed more security responsibility and Petraeus seeks to ensure that the gains achieved over the past several months continue.

With the exception of Petraeus, senior commanders generally arrive and depart with their units, which means that most of those now leaving or preparing to leave have been there for up to 15 months.

Topping the list of departures is Petraeus' second-in-command, Army Lt. Gen. Ray Odierno, who is due to leave in February when the 3rd Corps finishes its command tour and returns to Fort Hood, Texas. He will be replaced by Lt. Gen. Lloyd J. Austin III, commander of 18th Airborne Corps, from Fort Bragg, N.C.

Like many of the arriving commanders, Austin has extensive Iraq war experience. He was assistant commander of the 3rd Infantry Division when it led the invasion in March 2003 and captured Baghdad a month later. After a stint in Afghanistan, he was chief of staff at Central Command headquarters, which oversees all U.S. military operations in the Middle East, including the Iraq war.

Increasingly, Army and Marine commanders are focusing on non-combat aspects of the Iraq conflict - promoting economic growth, mentoring Iraqi forces and encouraging local, provincial and national political leaders to work out power-sharing arrangements and build civil institutions.

That is far from meaning that the danger is over. In Baghdad yesterday, a suicide car-bomber struck in one of the capital's most heavily guarded neighborhoods, killing two guards at a checkpoint near the home and offices of two prominent politicians, including the first prime minister after Saddam Hussein.

Both politicians were out of the country at the time.

The explosion took place in a neighborhood bordering the U.S.-protected Green Zone in western Baghdad, less than a quarter-mile from buildings that included the home and office compound of Ayad Allawi, a secular Shiite, and offices of Saleh al-Mutlaq, the head of the Iraqi National Dialogue Front, a Sunni political bloc.

It was the second bombing in two days to strike guards of Allawi, who is on a short list of possible future national leaders and a fierce critic of Shiite Prime Minister Nouri al-Maliki. *