Baghdad -- Another Marine died Saturday.

He was the fifth member of the 3rd Battalion, 4th Marines to die in the war on Iraq. But this was different.

This Marine didn't die fighting for a military objective, on the offensive. He was sitting on a tank, guarding a hospital, when an Iraqi militia fighter shot him in the back.

"That's harsh, man, but that's war," said another Marine in the same unit.

The Marines have entered a new phase in this war. It's about security and civil affairs and trying to help the local populace. But it's no less deadly.

Militia fighters and members of the Fedayeen Saddam continue to pop up, firing shots at U.S. troops and laying ambushes.

Throughout Baghdad, the sounds of rifle shots can be heard here and there. Usually just a pop or two. Sometimes a burst of automatic fire. Every once in a while, an explosion that rattles windows and knocks paintings on the wall askew.

Saturday afternoon, one or more Iraqis set up a machine gun in a house on the south side of the Tigris River, across from two hotels housing the international press corps. They opened up on the opposite bank and Marines returned fire. No Marines or hotel guests were hurt, but the Iraqis were either killed by .50-caliber machine gun fire or scared away by it, because the firing stopped.

A SHIFT IN DUTIES
Meanwhile, the Marines -- who are on the east side of the city, while the army is in the west -- are now being used as security forces and conducting civil affairs projects. They're supposed to work with local community leaders to provide security, stop the widespread looting and help with whatever problems arise.

Right now, that primarily means getting electrical power restored and water running. It also means providing security at local hospitals, which are still treating many civilians wounded in the fighting.

"We can't be seen as 'ugly Americans,' " Lt. Col. B.P. McCoy, commanding officer of 3rd Battalion, 4th Marines, told his staff. "It's all about attitude. Their psyches are a little hurt. A foreign army had to come bail them out. So let's help people and try to turn this thing around."

It's quite a shift for this battalion. They were very aggressive, seeing a lot of action with Iraqi military units and militia from Basra to Baghdad. The battalion, normally stationed at Twentynine Palms (San Bernardino County), was the first into central Baghdad and helped liberate this section of the city. They were the ones who helped topple the statue of Saddam Hussein in the square next to the Palestine and Sheraton hotels, homes of the press corps.

Now, instead of kicking in doors and tossing hand grenades, they're knocking on doors and taking tea with Iraqis.

"We will go from hero liberators to despised occupiers," McCoy said. "Whether it takes a year or a week depends on how we conduct ourselves."

It used to be that the Marines shot at any Iraqi holding a weapon. Now they're told to allow some Iraqis to keep weapons if the guns are for defensive purposes. The Americans are supposed to fire only if the Iraqi with the AK-47 acts hostile.

For those Iraqis who pose any kind of a threat, McCoy told his men to employ "a continuum of force" -- a verbal warning, physical restraint, butt- stroke (that is, striking with the butt end of a rifle) and then gunfire.

WAR TO DIPLOMACY
"We need to develop relationships with people and keep them up," said Maj. Martin Wetterauer, the battalion's operations officer. "We have to establish points of contact with the civilian population and help them. We need to make sure we keep our word if we say we're going to do something."

Some of the Marines privately grumbled about the new task. They say they feel like sitting ducks, just waiting for a militia fighter or Fedayeen to pop up and take a shot, like the one who killed the Marine on the tank.

His name is withheld pending notification of his family. But news of the shooting hit the battalion hard. There were a lot of grim faces as the men cleaned their weapons and ate MREs Saturday morning.

The worry about attacks struck home a couple of days ago with the discovery of a suicide bomb-making site. Marines from a sister battalion discovered a home that was full of suicide vests. They were made of leather and had explosives and ball bearings laced into the material.

A day before, a suicide bomber had attempted to kill some members of another Marine unit. The bomber jumped from a crowd and pulled a cord from under his clothing. He was cut in half and four Marines wounded by flying debris.

This is the new reality for troops in Baghdad.

"We can expect small-scale attacks on targets of opportunity," said Capt. Bryan Mangan, the battalion intelligence officer. "The strength of the militia is that they know the terrain. Their weakness seems to be that they're poorly trained and they have no command and control."

Col. Mike Marletto of San Leandro, commanding officer of the 11th Marine Regiment, said the hard part of the war has begun.

CHANGING ENVIRONMENT
"It's easy to come up and do what we do best -- fight a war," said Marletto, whose regiment took over security for the area around the Palestine Hotel, recently vacated by the 3rd Battalion, 4th Marines. "Now, our environment is changing every day and we have to adapt."

Marletto said the Marines expect to make major improvements in security for Iraqi civilians in the coming week.

Wetterauer, the operations officer, said Baghdadis are dropping off wounded family members and friends at Marine checkpoints, in order to get adequate medical care. And sometimes they drop off dead bodies, because they may have no other place to put them.

He said the new task for the Marines is different than they're used to, and requires a change of mind-set.

"This is something we're capable of doing," Wetterauer said. "But a lot will depend on the atmosphere. If the civilians are happy, they'll work with us -- and that will improve security for us, because they'll point out the troublemakers and trouble spots."

E-mail John Koopman at jkoopman@sfchronicle.com.

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