U.S. and British ground forces punched into Iraq across a broad front tonight after a booming artillery barrage, seizing territory along the Kuwaiti border with only modest resistance and pushing on toward the key southern city of Basra. While the sweeping land invasion began under a hazy desert moon, a second torrent of U.S. cruise missiles destroyed several buildings in Baghdad.

The long-awaited ground war started a day earlier than planned because of President Bush's decision to launch the "decapitation" attack on the Iraqi leadership early this morning, U.S. officers said. Although the invasion was clearly underway after months of buildup, U.S. defense officials characterized the movements as the first step in a much more massive push toward President Saddam Hussein's headquarters in the Iraqi capital.

In strikes designed to weaken Iraq's leadership, 24 Tomahawk missiles were launched tonight against Baghdad-area strongholds of the Special Republican Guard, Iraq's most elite military unit, as well as at the grounds of Hussein's main presidential palace and offices used by Deputy Prime Minister Tariq Aziz. The Tomahawks followed a 36-missile volley early this morning aimed directly at Hussein and his top lieutenants.

Iraq retaliated by firing back at the U.S. invasion force assembled in Kuwait, sending missiles southward intermittently throughout the day and into the night; at least three of the missiles were intercepted by U.S. Patriot antimissile batteries. Soldiers and Kuwaiti civilians alike returned repeatedly to shelters, but no casualties were reported.

[The first known U.S. or British military casualties were reported early Friday, however, in the crash of a Marine CH-46 Sea Knight helicopter in the Kuwaiti border area just south of the Iraqi port of Umm Qasr, at the head of the Persian Gulf. Marine officers said the aircraft, carrying four U.S. crew members and eight British Royal Marines, went down after encounter- ing haze from burning oil as it sought to reinforce a British position on the Faw peninsula. All aboard were reported killed.]

The ground assault began early this evening with a massive burst of 155mm artillery, mortar and rocket fire that rumbled for hours across northern Kuwait, shaking houses miles away and prompting eager U.S. soldiers to cheer as the shells flew overhead.

Soldiers from the Army's 3rd Infantry Division poured across the border around 8 p.m. (noon EST) at the western edge of the advance. To the east, the Marine 1st Expeditionary Force moved about the same time to seize control of and protect Iraq's southern oil fields, where several wells were reported to be ablaze. At the eastern edge of the invasion arc, on the swampy Faw peninsula, U.S. Navy and British commandos seized oil shipping and pumping facilities along the Persian Gulf.

Senior Marine officers reported ineffective resistance and no American casualties. But while the regular Iraqi army units defending southern Iraq have been described as weak and prone to surrender, they did not appear to be laying down their arms en masse at the initial contact. Front-line Marine and 3rd Infantry Division units reported engaging a few Iraqi infantry and tanks; officers said the clashes killed 14 Iraqi soldiers and destroyed 14 armored vehicles.

"Right now, they're fighting, not surrendering," a senior Marine officer said.

The U.S. forces did not detect the use of chemical or biological weapons by Iraqi units, officers said, but they nevertheless forged into Iraq wearing their full protective suits and toting gas masks. Similarly, the handful of Iraqi missiles fired into Kuwait carried conventional warheads, officials in Kuwait reported.

The push into Iraq and the airstrikes in Baghdad formally began a U.S. military campaign -- Operation Iraqi Freedom -- aimed at removing the Baath Party government and Hussein, who has ruled Iraq with ruthless efficiency for three decades and has long defied international demands that he cease efforts to build weapons of mass destruction.

Unlike the U.S.-led campaign in 1991 to expel Iraqi troops from Kuwait, the conflict that began today has failed to win endorsement from the U.N. Security Council or many foreign governments normally allied with the United States. But it has been cast by Bush as a necessary endeavor to destroy what his administration says is a stockpile of chemical and biological weapons -- arms that Iraq insists it does not possess.

Tonight's movement of M1 Abrams tanks, M2 Bradley Fighting Vehicles and other military hardware across the sand berms that separate Iraq and Kuwait had had an appearance of inevitability for months. U.S. commanders have steadily assembled more than 250,000 troops in the Persian Gulf region but found their war plans complicated by the Bush administration's failure to win U.N. backing and Turkey's refusal to permit U.S. forces to mount a northern front from its soil.

Restrictions imposed by the U.S. military barred reporting precise numbers or locations of the advancing forces. But officers have made clear in the preceding days that the legions of U.S. soldiers who entered Iraq, along with hundreds of thousands of troops still in Kuwait, intend to fan out across the desiccated landscape of southern Iraq over the next few days and push north to Baghdad to capture or kill Hussein -- unless air attacks get him first or spark a coup d'etat by his military.

If they succeed in changing the Iraqi government, the ground forces will face the challenge of winning over a population deeply skeptical of U.S. motives, rebuilding a war-torn country of 23 million people and holding together a society riven by religious, ethnic and tribal differences.

After the withering artillery barrage, the ride "across the line" began for several thousand soldiers in the Army's 3rd Infantry Division when they got the all-clear to head through an Iraqi minefield cleared first by engineers: "Objective cleared. No enemy," came the word from a Bradley Fighting Vehicle.

And with that, the division's 2nd Battalion plowed into Iraq, through the deserted demilitarized zone left over from the 1991 Persian Gulf War. Although there was little resistance, units of the division's 3rd Battalion, 15th Infantry Regiment reported one Iraqi soldier killed at one observation post and five at another.

Marines from the 1st Light Armored Reconnaissance unit of the 1st Marine Division also crossed the border at 8 p.m. and headed north, the first of a wave of Marines moving into Iraq. By 9:48 p.m., the Marines had their first reported contact with the enemy as they engaged a unit of Iraqi infantry and armor. Officers reported that the Marines destroyed a T-55 tank and were maneuvering to hit another.

The Marine commander, Lt. Gen. James T. Conway, accelerated his attack by four hours this afternoon after U.S. satellites detected burning oil wells in southern Iraq. Nine wells and pipeline sections were on fire tonight, according to Marine officers, but not the more critical and harder to rebuild pumping stations. Iraqis also ignited many trenches filled with oil, creating wafting plumes of smoke that obscured visibility for U.S. pilots and interfered with laser-guided munitions.

"There's oil trenches burning all over southeastern Iraq right now," said a senior officer.

While the Marines headed over the border, teams of Navy SEALs and British Royal Marines staged helicopter raids on two oil terminals in the Persian Gulf, Kaabot and Mabot, seizing both, Marine officers said. The Navy and British forces, operating under Conway's command, reported taking 15 detainees at one of the terminals.

Shortly afterward, other SEALs and British troops staged an air assault on an oil manifold and metering station on the tip of the Faw peninsula and captured that as well, officers said. The assaults were aided by Marine and British artillery based in Kuwait, fire from U.S. and British warships in the Persian Gulf, and air attacks by AC-130 Spectre gunships and A-10 Warthog attack planes.

Explosions were heard and flashes of light were seen late tonight in the direction of Basra, according to a Reuters reporter in northern Kuwait.

By capturing the two terminals and the manifold, U.S. military officers said they would be able to close off the valves and avert an environmental disaster similar to that caused in 1991 when Iraqi forces emptied oil into the Persian Gulf. The officers estimated most of the environmental damage caused 12 years ago could have been prevented had those facilities not been in Iraqi hands.

While the border was not breached until this evening, ground fighting began far earlier in the day as U.S. units moved up to the Iraqi border and prepared to breach the line of sand berms, trenches and electrified fence. "There's been a number of small engagements along the border all day -- mortars, machine guns, nothing big," said Col. John Coleman, chief of staff for the 1st Marine Expeditionary Force.

At 6:30 p.m., the Marines called in an airstrike on a 550-foot hill outside the Iraqi border town of Safwan to destroy an Iraqi observation post. U.S. warplanes dropped 11 JDAM precision-guided bombs on the hill.

The fires in oil wells and pipelines generated fears that Hussein's government was fulfilling threats to blow up Iraq's oil fields to deny the U.S. government an asset it hopes will fund postwar reconstruction. Reports of oil fires in Iraq, which is home to the world's second-largest crude reserve, sent oil prices surging, even after the Organization of Petroleum Exporting Countries tried to calm markets by pledging to speed output to make up for any disruption in Iraqi exports.

The first blow in Iraq's retaliatory missile strikes, identified by military officials as a CSSC-3 Seersucker cruise missile, slammed into the desert at 10:28 a.m., outside Camp Commando, the main headquarters for the 1st Marine Expeditionary Force about 25 miles south of the Iraqi border.

At least three missiles were apparently intercepted by Patriot missile batteries, U.S. officials said. In one case, an Army battalion commander said, three Patriot missiles were required to shoot down one of the Iraqi missiles because of an unspecified malfunction. But in general, the commander said that the new generation of Patriots performed better than the much-hyped but relatively ineffective Patriots used 12 years ago during the Gulf War.

Military officials said the missiles fired later in the day were either Ababil-100s or Al Samoud-2s. Iraq's possession of the Al Samouds, a newer and more powerful version of the Ababil, has been banned by the United Nations because their range exceeds a 93-mile limit imposed by the U.N. Security Council after the Gulf War. U.N. weapons inspectors had been supervising Iraq's destruction of more than 60 Al Samoud-2s before the teams were ordered out of Iraq on Monday.

One of the Iraqi missiles, which U.S. official believe was launched from the city of Basra, streaked over Kuwait and crashed into the Persian Gulf.

About 16 hours before the ground invasion began, the first strikes of the war soared from U.S. warships in the seas near Iraq. Thirty-six Tomahawk cruise missiles streaked from two destroyers, two cruisers and two attack submarines positioned in the Persian Gulf and Red and Mediterranean seas. They were launched as the moon was still shining; the missiles hurtled through the air roughly an hour before dawn.

On the USS Abraham Lincoln, the lead aircraft carrier in the Gulf, the start of the war surprised most of the crew. Many of the carrier's 115 fighter pilots were still asleep and an advancement exam that had been scheduled weeks ago was administered to hundreds of enlisted sailors.

"They started the war and forgot to tell us," quipped one of the pilots in the squadron that operates F/A-18E Super Hornets, the Navy's newest warplane. "This isn't the beginning yet -- that's when Baghdad gets all lit up with lights," said Lt. Rob Kihm, 28, who piloted a Super Hornet patrol over southern Iraq during the morning and returned to the ship without dropping any bombs.

Even high-level commanders were taken unaware by the early start. The early morning decapitation attack on Baghdad was launched in such secrecy that Lt. Gen. William S. Wallace, who as V Corps commander is the senior U.S. Army officer in Kuwait, was said by an Army source to have learned of the attack while watching CNN.

The results of the morning strike were unclear. Hussein appeared hours later on national television to castigate the U.S. military campaign.

With the air war launched early, the ground attack was advanced by 24 hours in an order issued this morning, the Army source said, with some quick shifts required as a result. The 101st Airborne Division had planned to take two days to assemble its ground assault convoys before heading into Iraq, for instance, but that timeline was cut in half.

As seen from a UH-60 Black Hawk helicopter late this afternoon, the Army convoys headed toward the border stretched nearly from horizon to horizon: open-sided trucks packed with infantrymen, heavily laden Humvees, 5,000-gallon tankers, 2,500-gallon tankers, earthmoving equipment, military vans, 18-wheel trucks, even leased Kuwaiti buses.

Several breakdowns caused delays for one 101st Airborne convoy, and there was a brief snarl when tanks and armored personnel carriers from the 3rd Infantry Division cut past unarmored vehicles from the 101st Airborne.

Senior officers speculated that the acceleration was intended to maintain the momentum of the attempted decapitation attack. That would be an echo of the Gulf War of 12 years ago, when the main ground attack by the Army's VII Corps was accelerated by nearly 24 hours on orders from Gen. H. Norman Schwarzkopf in an effort to capitalize on the early success of his preliminary forces.

Correspondents Rick Atkinson, Peter Baker, William Branigin and Lyndsey Layton with U.S. forces and staff writers Bradley Graham and Thomas E. Ricks in Washington contributed to this report.

The Planning Ministry in Baghdad erupts in flames after being hit during a second round of U.S. cruise missile strikes against the Iraqi capital. Soldiers from the British army's 29th Commando Regiment Royal Artillery fire 105mm guns from a position in the Kuwaiti desert at targets in southern Iraq. Marine Lance Cpl. William McNeal of Florida secures the perimeter of the 2nd Tank Battalion west of Basra, in southern Iraq. The unit arrived in the area today, after a 12-hour drive from Kuwait.