Updated

Combat in Iraq is all but over, yet U.S. Army troops continue to stream into the country in an illustration of one of the ironies of current warfare: More soldiers often are needed to keep the peace than to fight the war.

The size of the U.S. military force in Baghdad dropped significantly this past weekend when Army soldiers took over from Marines in eastern parts of the Iraqi capital.

But those Marines didn't leave Iraq. They moved south to bolster other Marines providing security and humanitarian assistance in southern Iraq. Army units, including ones continuing to arrive in Iraq, will do the same in Baghdad and northern Iraq.

Top priorities for the U.S. military in Iraq now include searching for weapons of mass destruction and former Iraqi officials, as well as setting the conditions for a new Iraqi government to come to power.

Last week, Defense Secretary Donald H. Rumsfeld described the military's role not as "nation building" but as helping in the transition from the overthrown government to a new one.

"What has to happen is, somebody has to try to create an environment that's sufficiently secure and hospitable to that kind of a change, but doing it without doing it in a manner that creates a dependency," Rumsfeld said in a town hall-style meeting with Pentagon workers.

Such work is called "Phase IV" or stability operations in military parlance. Much of the work will be done by the same troops that fought the war.

Other units called "civil affairs" forces specialize in these operations, and some accompanied the combat troops into battle. They're under the same military chain of command but specially trained and equipped to get food, water, shelter and medical care to needy populations and keep local rivalries from flaring into more violence.

U.S. allies also will help with the Phase IV efforts, Rumsfeld said Monday. Italy, for example, is sending 300 police, while the Czech Republic, Spain, Lithuania and Jordan are giving medical aid, he said.

There are civil affairs units spread throughout the military, and regular troops also have some training in such tasks. Still, civil affairs units have been among the most heavily used in the military, including most recently in Afghanistan. Rumsfeld said that fact could lead to changes in the military's structure.

"At the present time, an awful lot of the civil affairs people are in the reserves, which is not a good thing," Rumsfeld said last week. "If you're going to need those skills, you need some on active duty. You can't have them all in the reserves, or else you're going to call people up every other year, which isn't what they really sign up for."

Stability operations usually are not as dangerous as combat, but they can be just as demanding and require much more finesse.

"We're in a different phase now that is going to be a little tougher," said Air Force Lt. Gen. T. Michael Moseley, commander of the air war in Iraq. "The Army has a tough job ahead of them. They've got to stabilize Iraq and make it livable."

Soldiers in combat can rely on airstrikes against command and communications targets, as well as enemy troops. The work of policing and delivering aid, by contrast, must be done on the ground. That's a major reason why providing security and humanitarian aid can require more troops than actual combat.

For example, getting the electricity back on in Baghdad has required engineering to get power plants running as well as working with technocrats from Saddam Hussein's former regime who were in charge of the power infrastructure. In securing the capital, U.S. armored vehicles have escorted Iraqi police patrols, and U.S. troops have protected hospitals and other sites from looting.

An illustration of what can go wrong came April 10 in Najaf, a southern city that's one of the holiest sites in the world to Shiite Muslims, who make up a majority of Iraq's population. Two Shiite clerics -- one who had supported Saddam's regime, the other the son of a supreme ayatollah Saddam had persecuted -- were to meet in reconciliation.

Eager to publicize the symbolic meeting, the U.S. military flew a helicopter load of journalists to witness the encounter. But an angry crowd hacked both clerics to death just before the reporters arrived.

The Army's 4th Infantry Division, the latest to arrive in Iraq, is helping patrol Baghdad with the 3rd Infantry Division and elements of the 101st and 82nd Airborne divisions. Elements of those Army divisions also are working in northern Iraq, along with special operations forces and members of the 173rd Airborne Brigade.

The 3rd Infantry and airborne elements -- the first Army units into Iraq -- won't be able to leave for at least several weeks, military officials say. The Army's 1st Armored Division is preparing to enter Iraq in the meantime, though without its now unneeded artillery.