U.S. Army troops sorted out enemy fighters from friendly civilians in training exercises Monday designed to help them with one of the most dangerous forms of combat they could face in Iraq -- urban warfare.

After conducting sweeping tank maneuvers a month ago, the soldiers worked out tactics for taking buildings room by room -- deciding in a split-second whether people inside should be killed.

The desert-tested soldiers of the 2nd Brigade, 3rd Infantry Division are being joined by tens of thousands more troops as President Bush ponders whether to invade Iraq.

U.S. military planners believe Saddam Hussein might forgo battle in the open desert and instead fall back to Baghdad. Concentrating his defenses in urban areas would put U.S. troops in more danger and risk massive civilian casualties if air power is brought to bear.

Task Force 315 of the 2nd Brigade began honing its urban fighting skills this month, training at an abandoned mining camp and a mock village in the Kuwaiti desert. The drills included live-fire movement in obstacle-laden streets, and mock room-to-room skirmishes.

The training was initiated by the task force sergeant major, Robert Gallagher, 41, of Toms River, N.J., who is a 21-year veteran. He applied some of the lessons he learned on a rescue mission in one of the Army's most disastrous urban war experiences -- the battle in Mogadishu, Somalia in 1993 when two Black Hawk helicopters were brought down and 18 U.S. servicemen killed.

"The biggest thing I want to do is get people to move around tanks and buildings, competently and confidently," Gallagher said. "Can we shoot and move in the desert? Yes. Can we shoot and move in an urban environment? Yes. The American people have a right to expect us to fight to the standard they demand."

On Monday, the soldiers of Alpha Company added realism to a skirmish when they had to decide a split-second after kicking open doors -- at least, those that weren't booby-trapped -- whether they were facing combatants or civilians.

Initially, several "civilians" were killed, an expected result that led to question-and-answer sessions with the company's leaders.

"What are civilians going to do?" company commander Capt. Josh Wright asked.

"Run," came one answer.

"Fight and die," came another.

"Most real true civilians are going to hunker down and take cover," Wright explained. "They're going to be terrified."

Civilians may have to be forced to the ground and even handcuffed if necessary, but shouldn't be injured, the soldiers were told.

The soldiers acknowledge how much real life can differ from training.

"There are fundamental differences in attacking a camp like this and a real downtown area," acknowledged Wright, 29, of Girard, Ill. "You would have all different kinds of things in a city -- people at a market, mothers taking their kids to school."

But he said that the Army had become accustomed to operating in urban environments in Bosnia, Kosovo, Afghanistan and Somalia.

On Sunday and Monday, platoons in A Company traded off playing roles attacking and defending a mining camp at night. The attacking forces rushed up the flanks of the buildings in fast-moving Bradley fighting vehicles.

The Bradleys dropped their rear hatches and disgorged squads of troops who took up position on the outer walls.

"Window right!" came the command. "Frag it!"

A soldier pretended to lob a grenade into a window and a couple of soldiers entered sweeping the room with flashlights and M-16 assault rifles. From other buildings, defenders opened fire with blanks, touching off short firefights as attacking forces cleared other buildings room by room.

A trio of defenders in a corner room of one the last, sand-covered buildings held out until their door was kicked in and attackers jumped through a hole in the wall.

"I love you Americans!" joked Pfc. Josh Vasquez of Canton, Ohio, raising his hands. "I surrender!"

"In reality we had been dead 20 minutes before -- the tanks and Bradley's would have taken us out," he said later.