BAGHDAD, Iraq, April 8 — When the United States invaded Iraq a year ago, one of its chief concerns was preventing a civil war between Shiite Muslims, who make up a majority in the country, and Sunni Muslims, who held all the power under Saddam Hussein.
Now the fear is that the growing uprising against the occupation is forging a new and previously unheard of level of cooperation between the two groups — and the common cause is killing Americans.
"We have orders from our leader to fight as one and to help the Sunnis," said Nimaa Fakir, a 27-year-old teacher and foot soldier in the Mahdi Army, a Shiite militia. "We want to increase the fighting, increase the killing and drive the Americans out. To do this, we must combine forces."
This new Shiite-Sunni partnership was flourishing in Baghdad on Thursday. Convoys of pickup trucks with signature black Shiite flags flapping from their bumpers hauled sacks of grain, flour, sugar and rice into Sunni mosques.
The food donations were coming from Shiite families, in many cases from people with little to spare. And they were headed to the besieged residents of Falluja, a city that has now become the icon of the resistance, especially after the bombing on Wednesday of a mosque compound there.
"Sunni, Shia, that doesn't matter anymore," said Sabah Saddam, a 32-year-old government clerk who took the day off to drive one of the supply trucks. "These were artificial distinctions. The people in Falluja are starving. They are Iraqis and they need our help."
But it is not just relief aid that is flowing into the city.
According to several militia members, many Shiite fighters are streaming into Falluja to help Sunni insurgents repel a punishing assault by United States marines. Groups of young men with guns are taking buses from Shiite neighborhoods in Baghdad to the outskirts of Falluja, and then slipping past checkpoints to join the action. "It's not easy to get in, but we have our ways," said Ahmed Jumar, a 25-year-old professional soccer player who also belongs to a Shiite militia. "Our different battles have turned into one fight, the fight against the Americans."
American leaders had been concerned that the rival sectarian groups would not find a common cause. Now, it seems, they have found a common enemy. "The danger is we believe there is a linkage that may be occurring at the very lowest levels between the Sunni and the Shia," Lt. General Ricardo Sanchez, commander of the occupation forces, said on Thursday. "We have to work very hard to ensure that it remains at the tactical level."
He also said the call for unity is "clearly an attempt to take advantage of the situation."
Brig. Gen. Mark Hertling, an assistant commander of the First Armored Division, said military intelligence indicated that there might be some loose coordination between the renegade Shia movement of Moktada al-Sadr and a Sunni extremist group called Mohammed's Army in the western portions of Baghdad.
He said troops from the First Armored and the First Cavalry Divisions were conducting reconnaissance and offensive operations against fighters from both groups, who have converged on the road to Falluja.
The city, 35 miles west of Baghdad, has become the rallying cry of the resistance. It is in its fifth day of siege. Marines are trying to root out insurgents after four American security guards were ambushed there last week and their bodies were mutilated by a mob. American troops have been fighting house to house and mosque to mosque against a determined group of guerrillas. According to people inside Falluja, the situation is grim and getting grimmer.
"It's a disaster," said Sheik Ghazi Al Abid, a wealthy tribal leader, who was reached by telephone. "There's no food, no water, no electricity."
The sheik said it was so dangerous that bodies have been left on the streets because people are terrified to venture outside to collect them.
"Anybody who moves will get shot," the sheik said. "We need all the help we can get." He also said more than 300 people had been killed, hundreds more had been wounded, and medical supplies and blood were running low." There are so many injured civilians," the sheik said, "they don't know where to go."
In Baghdad, blood banks were packed. Imams at both Sunni and Shiite mosques put out a message that Falluja residents needed blood fast. On Thursday, a group of Shiite men formed a line at one Baghdad blood bank that wended out the door. The men were ready to get pricked with a needle for their Sunni brothers. "We share a cause now," said Mohammed Majid, a taxi driver. "Why not share our bodies?"
Pentagon officials said Thursday that they had no definitive figures on the size or scale of the Sunni or Shiite militias. That is largely because the militia movement seems too fluid, and it is splintered among several factions. "It's a mob mentality," said one intelligence official. "They are recruiting among a lot of unhappy people."
Shiite extremist groups have a long tradition of hiding their true strength, in large part because their history has been marked by persecution by Sunni elites in many Muslim countries. In southern Lebanon in the 1980's, for example, the Central Intelligence Agency was never able to get solid estimates of the number of Shiite fighters involved in Hezbollah or the Islamic resistance that eventually forced the Israeli withdrawal from Lebanon, former United States intelligence officials said Thursday.
Those former officials pointed out that the practice of Taqiyya — dissembling about one's religion, especially in times of danger — is particular to Shiism. That secretive tradition has made Shiite groups extremely difficult for intelligence officers to penetrate, the former C.I.A. officers said.
Until last week, the Shiite groups had mostly sat out the resistance. Many Sunni fighters were loyal to Mr. Hussein. That alienated Shiites, who had been ruthlessly persecuted by the former Iraqi leader.
All that changed this week when Mr. Sadr activated his militia at the same time Falluja faced its biggest battle. Now, the two sides have joined. There were even reports on Thursday of armed men from Falluja escaping to Shiite neighborhoods in Baghdad. Mr. Hussein is no longer mentioned. Fighting the infidels is.
James Risen contributed reporting from Washington for this article.
© Copyright 2004 The New York Times Company