Under the shelter of night, and with the support of heavy bombing, the 1st Marine Division entered Iraq at around 9 p.m. local time (1 p.m. EST). Traveling north in their armored vehicles, the Marines encountered some resistance from "rear guard" units; they opened fire with machine guns on an Iraqi T-55 tank and destroyed it with a Javelin, a portable anti-tank missile.

There were no American casualties.

A reporter for The Times of London reported that Royal Marine Commandos had also crossed into southern Iraq. According to this account, hundreds of British troops had attacked "Red Beach" at the head of the Persian Gulf.

The British Marines were supported by a bombardment across the Khawr Abd Allah, the river estuary that separates Bubiyan Island in Kuwait from Iraq, according to the report.

By taking southern Iraq, the allies would command access to the Persian Gulf and set the stage for the first major conquest on the way to Baghdad ­ Basra, Iraq's second largest city, just 20 miles from the Kuwait border.

None of the forces apparently encountered chemical or biologicial weapons. But the Marines passed oil wells, possibly set on fire by the Iraqis. Flames shot hundreds of feet in the air, and the air was thick with black smoke.

Artillery, mortars and howitzers rumbled for hours in the nearly deserted far north of Kuwait, mixed with bursts of rocket launchers. The explosions rattled tin roofs noisily on their wood frames miles away and shook concrete houses.

The attack came at the end of a day that began with allied troops at the other end of the gun barrel, as Iraq ­ responding to the American bombardment of Baghdad and other targets ­ launched missiles into Kuwait, where allied forces were primed to attack.

The Iraqi military claimed in a statement that it had repulsed an "enemy" attack at Al-Anbar province, on Iraq's border with Jordan and Saudi Arabia; it was not clear what force could be attacking from that point. It made no mention of the attacks at the tip of the Gulf.

The Iraqis did deny a Kuwait News Agency report that the city of Umm Qasr had fallen to U.S. and British troops and hundreds of Iraqi soldiers had surrendered.

Meanwhile, the U.S. 3rd Infantry Division's artillery opened fire on Iraq with Paladin self-propelled howitzers and multiple launch rocket systems. More than 100 artillery shells were fired toward southern Iraq in a five-minute barrage. White light glowed in the sky above the cannons, as explosions were heard from Iraq.

No fire was being returned.

Infantrymen, deployed between the howitzers and the Iraqi border, cheered as the 155 mm shells screamed overhead.

Their targets were not clear, although it appeared this was not the start of the pedal-to-the-metal offensive promised by U.S. Secretary of Defense Donald H. Rumsfeld on Thursday ­ an assault with a "force and scope and scale that has been beyond what has been seen before."

The shooting was unnerving to those within earshot, nonetheless. Foreign farm workers ran out in their yards in the dark, shouting. Pakistani and Indian farm workers shrank at each salvo. "Give me my passport," one field worker told his foreman.

"The Americans are bombing to the left of us, to the right of us, the front, the backside, and I'm under it!" the foreman said later.

Troops continued to stream toward the Iraq border. A huge convoy of trucks, tankers, humvees and every imaginable sort of military vehicle of the 101st Airborne Division rolled across the desert late Thursday night under a round white moon.

Troops in the backs of heavy trucks rode with scarves pulled up across their faces as huge clouds of dust rose from the flat surface. Pairs of red tail lights and yellow headlights strung across the desert, filtered by a fog of dust.

The convoy moved at a steady clip of about 30 mph, in a constant rumble of humvees and the grinding of huge tankers

The troops were largely silent, getting down occasionally to stretch their legs when the convoy stopped to wait for any vehicles straggling in soft sand areas.

Earlier in the day, the troops in waiting had their first brush with action when Iraq fired missiles into Kuwait. There were cries of "gas, gas, gas," and U.S. troops were sent scurrying for their protective suits and gas masks ­ for naught, as authorities said none of the missiles carried biological or chemical payloads.

Soldiers of A Company, 3rd Battalion, 7th Infantry Regiment were eating lunch when an Iraqi missile hit the desert. They wore the masks for 20 minutes until given the all-clear.

After removing his mask, the company commander, Capt. Chris Carter of Watkinsville, Ga., said: "Saddam is a fool."

"I think it's an obvious attempt by Saddam Hussein to demoralize the army and the American public," Carter said. "An attempt that has been a miserable failure. He's probably got the guys more ready to fight than ever."

The men of the unit returned to cleaning their weapons and reading books, waiting for their part of the war to begin with a new awareness of the hazards ahead.

"I know what I'll be using as a pillow tonight," Staff Sgt. Bryce Ivings of Sarasota, Fla., said of his protective suit.

After weeks on standby, U.S. troops were eager to get on with their mission.

Some Marines were simply excited to begin fighting, something they had trained to do for years, and occasional screams of "Let's get it on!" came from some of their weapons holes.

Others said they had no other way back to the United States but through Iraq.

"Vegas is that way," Cpl. Matt Nale, 31, of Seattle, said, pointing north to the border. "That's my way to Vegas."

Associated Press writer Christopher Tomlinson also contributed to this report.