WITH THE 3RD INFANTRY DIVISION, in central Iraq - A crude sign now stands at the checkpoint where four Americans died when a bomb in a taxi exploded Saturday.
"Roadblock ahead," it states, in Arabic. "Leave the area or we will fire."
On Saturday, the road was open to anyone who did not appear to be a threat to the American forces arrayed in the flat scrub desert of central Iraq. Yesterday, it was closed to everyone.
So were other roads leading into Najaf, a city on the Euphrates River now encircled by soldiers from the 3rd Infantry and the 101st Airborne divisions. The blockade was the most visible response yesterday to the bombing, which officers say they suspect was a suicide attack, the first against American forces since the war in Iraq began.
Restrictions on reporters traveling with American forces prohibit descriptions of "rules of engagement," the tactics used when American soldiers encounter enemy forces. But Lt. Col. Scott E. Rutter, commander of the 2nd Battalion, 7th Infantry Regiment, whose soldiers died in the attack, characterized the new rules in a blunt and unclassified way.
"Five seconds," he said yesterday, his voice still laced with anger. "They have five seconds to turn around and get out of here. If they're there in five seconds, they're dead."
The new restrictions effectively blockaded Najaf, a holy city for Shiites from Iraq and beyond, which American commanders had hoped would be a breeding ground for popular dissent against President Saddam Hussein's rule.
Now those Iraqis inside the encirclement, still controlled by security troops and militiamen loyal to Hussein, have lost their way in or out.
Maj. Gen. Buford C. Blount III, commander of the 3rd Infantry, said that tighter security measures were an unfortunate but necessary step to ensure the safety of his troops, but one that he acknowledged would increase the hardships for noncombatant Iraqis.
"We went into this hoping to keep collateral damage and civilian casualties to a minimum," he said in an interview yesterday at the checkpoint. "They've not let us do that."
The bombing came six days after units of the 3rd Infantry arrived at the broad, flat desert beyond the escarpment near Najaf - only to see their rapid advance stalled by skirmishing around the city that they had hoped to simply pass by.
The delay has angered and frustrated soldiers who feared becoming bogged down in clashes and attacks like Saturday's. Yesterday, three Iraqi missiles landed near the division, heightening anxieties, though they landed harmlessly.
After days of clashes with Iraqi fighters, the bombing exposed a seething anger among soldiers that seemed to strain efforts to portray the war as one against Hussein's government and not the Iraqi people.
"We really are trying to maintain our professionalism," said Capt. James K. Lee, the commander of the soldiers' company. "We don't want to discriminate against the Iraqi people. If we do that, we lose the war for the hearts and minds of these people."
At the checkpoint, the consequences of the new restrictions were nonetheless evident. American bulldozers uprooted palm and eucalyptus trees along the road and leveled a two-story home to clear fields of fire for troops on guard.
A few hundred yards up the road was the burned-out shell of a car that failed to heed the new sign to go no farther. A second car, with a man and a woman, also tried to bolt through the checkpoint early yesterday. Soldiers opened fire and killed the man.
"His wife watched him die," Rutter said.
At the time of the bombing, Saturday morning, the checkpoint was not bustling exactly, but fairly crowded, according to the soldiers who were there.
A minivan full of passengers had just been turned around and was waiting on the shoulder while troops debated whether to send them back north, where they came from. A man in a white pickup had also been stopped, but he refused to leave, sitting in the median between the northbound and southbound lanes and claiming his leg was injured. A man on a bicycle was pedaling by.
It was then that the taxi approached. The driver was in his 40s or 50s, with dark skin and a mustache, dressed in civilian clothes. The four soldiers who died surrounded the car and ordered him out for a search.
The taxi exploded when the man followed orders to open the trunk.
Sgt. Rohan A. Green, a medic, was in an armored ambulance when the bomb exploded. There was nothing he could do for the soldiers. He said death evoked different reactions, depending on how close one was to it.
For his part, he said simply: "Everyone wants to go home. They want to go home alive."