Days of heavy U.S. air strikes and artillery fire gave way after dark Monday to an invasion of Fallujah by as many as 15,000 American Marines and soldiers, backed by Iraqi troops as well as tanks and armored vehicles. Reporters embedded with coalition forces describe a night of intense ground fire with U.S.-led forces moving into Fallujah hours after Iraqi Prime Minister Iyad Allawi gave the go-ahead for the ground assault to begin.

By telephone from Baghdad, the top American commander in Iraq, Army General George Casey, told reporters he expects a tough fight against an estimated 3,000 insurgents and terrorists who have turned the city into a base of operations ever since American Marines abandoned the fight to retake it in April. "Our estimates tell us that they will probably fall back toward the center of the city where there will probably be a major confrontation," he said.

While vowing that this time coalition forces will succeed in bringing law and order to the Sunni stronghold, Defense Secretary Donald Rumsfeld is warning the battle for Fallujah will not necessarily be the last showdown with insurgents. "It's a tough business and it's going to take time. And I wouldn't want to suggest that this one city [will be the last we are going to have to fight for]; it's important and it needs to be done and you obviously can't have terrorist safe havens in a country and expect the government to be able to function as a government must," he said.

The U.S. military acknowledges that among the thousands of Fallujah residents who have fled the city are terrorists and insurgents the military coalition is trying to capture or kill. A curfew has been imposed in part to minimize civilian casualties and coalition aircraft have been dropping leaflets warning people to remain indoors. "There aren't going to be large numbers of civilians killed, certainly not by U.S. forces," Secretary Rumsfeld said.

Fallujah is believed to be the base of operations for the terrorist network headed by Abu Musab al-Zarqawi, the Jordanian wanted for some of the deadliest attacks on foreigners in Iraq since the start of the war. At this point though, U.S. defense officials are not saying who, if anyone, may have been apprehended or killed in these early hours of the Fallujah invasion.