Highway 80 in Southern Iraq ? Allied forces crossed the Euphrates River and were halfway to Baghdad on Saturday, their swift advance unimpeded by lingering resistance in the cities of Basra and Umm Qasr. The biggest hurdle: moving the massive military machine across the desert.

Maj. Gen. Stanley McChrystal of the Joint Chiefs of Staff said forces had moved 150 miles into Iraqi territory. “The forces have moved with impressive speed thus far,” he said.

In the southern city of Basra, they faced artillery and machine-gun fire. So rather than risk a bloody urban battlefield in a city of 2 million, the allies took what they needed — an airport and a bridge — and moved on, leaving British forces behind.

“This is about liberation, not occupation,” Gen. Tommy Franks said.

Skirmishes — sometimes with stiff resistance — took place at the front end of the advance. Iraqi state television reported fighting between Iraqi ruling Baath party militias and U.S.-British forces near the Shiite holy city of Najaf, 95 miles south of Baghdad. It said the top Baath party official in Najaf was killed.

U.S. Army infantry engaged a daylong battle with Iraqi troops at the city of As-Samawah, downriver from Najaf and 150 miles south of Baghdad, as the Americans seized two bridges near the Euphrates’ southern bank.

Iraqi fire forced the Americans to pull back from the bridges for a time, until they called in a barrage of artillery fire and secured the crossings, an Army Times correspondent with the unit reported. Forty Iraqi soldiers were killed, but continued firing slowed the Americans’ advance Saturday evening.

Meanwhile, the U.S. Army’s V Corps took Nasiriyah, another major crossing point over the Euphrates northwest of Basra.

Further south, coalition troops were trying to mop up resistance at the Persian Gulf port of Umm Qasr so it can be used for humanitarian shipments. They faced street-to-street fighting against guerrillas, among them members of Saddam Hussein’s Fedayeen, the Baath Party paramilitary organization.

“It’s easy to sit in a window and fire a rifle,” said Lt. Col. Chris Vernon, a British military spokesman. He said some had changed into civilian clothing to blend in with the population and take advantage of allied desire to minimize civilian casualties.

“The Americans would actually say, ‘We’ve seen this guy, we let him go, and here he pops up again fighting,'” Vernon said.