American forces are still not clear exactly who their opponent is. Enemy fighters they have killed have not carried identification, and local residents have provided only limited intelligence about who is behind the attacks.

But one thing is already clear. American forces seem to be battling a small but determined foe who has a primitive but effective command-and-control system that uses red, blue and white flares to signal the advance of American troops. The risk does not come from random potshots. The American forces are facing organized resistance that comes alive at night.

Specialist William Fernandez experienced the enemy tactics firsthand while on patrol on Sunday night. The patrol was made up of National Guard troops from Massachusetts and Rhode Island who serve in the military police unit here. Specialist Fernandez, a computer engineer in civilian life, was operating the radio.

When he saw a red flare he sensed his patrol was about to be attacked. Suddenly, a grenade exploded directly behind the column of six Humvees, a move he believed was intended to encourage the Americans to drive forward into the kill zone.

Automatic-weapons fire erupted from several rooftops. The Americans fired at the muzzle flashes and left the scene after several minutes. Most of the Humvees had bullet holes, but the soldiers somehow escaped injury.

"It is a miniwar," Specialist Fernandez said.

Military officials do not know if the attacks in Falluja are coordinated by a single group or whether various factions are involved. Nor is it clear whether former Baathists are paying residents to fight the Americans or whether the attacks are being instigated by Arabs from other countries.

But what is certain is that some attacks are premeditated and involve cooperation among small groups of fighters, including a system of signaling the presence of American forces.

In Baghdad today, one American soldier was killed and another was wounded in a rocket-propelled grenade attack.

Falluja, a city of 200,000 about 40 miles west of Baghdad, is a largely Sunni city that was untouched by the war. When the 4,000 troops from the Spartan Brigade — officially the Second Brigade of the Third Infantry Division — and other associated units arrived last week, they encountered a wary population.

Residents have displayed little hostility to the formidable American force during its daytime patrols. The troops have even been cheered by schoolchildren, who seem fascinated by their body armor and Humvees.

Iraqi officials say the attacks are being carried out by what they call outsiders who slip into Falluja and attack at night. Evaluating that claim has been difficult. But there have been clear indications that many of the attacks are coordinated.

The adversaries use colored flares to signal the approach of American troops and apparently to signal which type of forces are approaching — a formidable armored column or a more vulnerable group of Humvees. A red flare, American soldiers say, signals that American troops will soon be within firing range. Sometimes people flash lights in buildings along the way to signal the advance of American forces.

American troops say they believe that the adversary's goal is not just to pick off American soldiers but also to provoke the troops to fire into civilian neighborhoods, a response that would build popular opposition to the American presence here.

The attacks appear to be conducted by small groups. One of the first attacks took place on Friday night when a patrol from the 3-15 infantry of the Spartan Brigade got close to a mosque. A man with an automatic rifle fired from the mosque while another attacker across the street fired a rocket-propelled grenade. The Americans killed the assailant in the mosque, established a cordon around the building and searched it.

An OH-58 helicopter flew near the mosque the next night to monitor the scene, and the pilot saw gunfire, which may have been directed against the helicopter itself.

The American troops from 3-15 infantry, who also defend the compound of the Falluja mayor, have been attacked on two occasions. In one episode, American snipers killed an off-duty policeman and wounded a security guard. The guard said he was looking for looters. Col. David Perkins, the commander of the Spartan Brigade, said that the men were seen trying to sneak up on American troops defending the compound and that one of the assailants fired first.

On Monday night, a patrol from the 4-64 Armor Battalion, which is stationed at the nearby town of Habbaniyah, was attacked when a mine exploded as one of its columns drove past. The explosion, set off by remote control, blew a hole three feet deep but missed the column.

While the town has been generally calm, there have been other attacks involving rocket-propelled grenades, small arms, automatic weapons and even a car, which an arms dealer used to try to run into American troops. So far, there have been no American casualties.

Specialist Fernandez's unit returned to the scene of its ambush the next day. A search yielded several AK-47's and some grenades. Traces of blood were found, too, a sign that the Americans might have wounded one of their opponents the previous night. Two suspects were detained.

Gauging the enemy's tactics is not always easy. One recent evening, when the National Guard troops were on patrol, all of the lights in the neighborhood suddenly went off. The soldiers were concerned that their foe had switched off the electricity and that an attack might be imminent. So they quickly left the scene.