As thousands of U.S. Marines arrive in Haiti to keep the peace, the job becomes increasingly difficult.

BY JOE MOZINGO

COL. DAVE BERGER, commander of the 3rd Battalion, 8th Regiment

PORT-AU-PRINCE -- Just as the morning sun finally beat the mosquitoes back -- and hours before U.S. Marines would fire their first rounds in this country -- Maj. Justin Rodriguez interrupted the morning briefing to lay out, in the ragged language of the Corps, a dictum for this mission.

''This ain't a war,'' he said. ``The Haitian people aren't the enemy.''

He told his company, hunkered down on a remote corner of the airport, that some soldiers were too aggressive.

``The other day we had a Marine trying to keep a gate closed at the port. Some kids kept coming up to the gate because they were curious.''

Rodriguez paused for a moment. And then he drew his voice deep and hard from his gut: 'We don't need a Marine aiming his weapon and yelling, `Get the - - - - out of here!' ''

Once again, U.S. soldiers find themselves in a situation where the enemy is undefined and largely unknown, blending into the very crowds peacekeepers are here to protect.

STARK OBSTACLES

Sunday's gun battle -- in which seven people died and some 30 were wounded in front of the National Palace -- showed the stark obstacles the soldiers will face in trying to get this country up and running again.

Before the shooting began Sunday, Col. Dave Berger -- commander of the 3rd Battalion, 8th Regiment -- said the chaotic yet delicate situation his troops are facing is real-time practice for Iraq, where they are scheduled to go next.

The mission Sunday: Secure the National Palace and the square in front of it for the first large protest against exiled-President Jean-Bertrand Aristide since he left Feb. 29.

From his position within the palace gates, Berger and other commanders scanned a crowd of thousands, who were dancing, chanting and circling the square. Sniper teams set up on the palace roof.

Word was out that a group of Aristide supporters -- and possibly the murderous chimres -- planned to crash the party. But no one knew whether they would come as one big gang or simply infiltrate the crowd. Berger and his boss, Col. Mark Gurganus, struggled to come up with a way to a defuse a gun battle without shooting innocent people.

''It's going to be hard,'' Berger said. ``One of the things we can do is take a light-armored vehicle into the crowds and do a slow circle to disperse them. It can't boil as long as you're stirring it.''

But it did boil as the protest was winding down about 2:30 p.m. Gunmen emerged in the crowd and from a building nearby and began shooting people. The Marines took fire themselves and shot one man dead.

In the aftermath, they are asking themselves whether they could have done more to prevent the incident.

''You got to be pretty discriminate about your shooting in a situation like that,'' Capt. Brad Cornali said.

''We're trying to encourage democracy,'' said a disheartened Berger at the end of the day. ``I'm afraid if we blanket the area, we'll prevent exactly what we're trying to generate.''

ROUTINE BEGINNING

Sunday started as usual, with soldiers popping malaria pills and scratching the bites they incurred throughout the night.

At around 9 a.m., the supply convoy lined up its vehicles. In a high-back Humvee, six Marines -- kids really -- argued the merits of Haiti over their last assignment, counter-insurgency work in the Philippines.

''This is heaven compared to the Philippines,'' said Denis Couture, 19, of Boston.

The convoy moved out around 10. As an indication of the difficulties the Marines face in simply getting their bearings in this country, the heavily armed unit had to follow a Haitian guide in a Dollar Rent-a-Car to navigate the city's maze of streets.

Sgt. Michael Haddle drove an armored ''hardback'' Humvee with a mounted .762-caliber machine gun, passing crowds of Haitians and overloaded colorful tap taps, pickups that serve as public transportation. Most stared impassively at the Americans, some waved, some shouted at them to go home. One man gave them the middle finger.

''The reaction to us, it really depends where you are,'' said Lt. Kyle Aldrich. ``I guess it's symbolic of the whole situation here.''

They arrived at the palace and set up a command post. Two Huey helicopters monitored the progress of protesters, who were marching down from the suburb of Petionville.

The morning was quiet. Many soldiers slept under the trees and ate their rations. Even when the protesters arrived, the event stayed peaceful until the very end. The shooting began around 2:30 p.m. Sometime soon after, soldiers saw people scattering and screaming on the northeast side of the palace. They pointed to two gunmen -- one holding a 45, one reaching for his belt.

The soldiers yelled at everyone to get down, and shot one of them, they said later. The other escaped.

SHOOTERS SCATTERED

The crowd began carrying the wounded to the front gates, and the light-armored vehicles rolled out. The Marines say they hunted shooters down, but they all scattered into the narrow winding alleys of the neighborhood.

At 5 p.m. Berger called everyone in a circle for a debriefing. Anyone involved in the shooting or subsequent manhunt took center stage and described what happened, as one Marine typed up the chronology on a laptop.

One soldier who went into the streets said he saw people using cellphones, wearing headsets and distributing American cash -- a possible indication that the attack was funded and well-coordinated.

As the mosquitoes came back out of hiding as the sun dropped over the mountains, some Marines moved into the palace barracks for dinner, and the supply company headed back down to the airport in the waning light.

''We will go back now and figure out how to do it better next time,'' Berger said.