Opening Attack of Iraq War: On March 20, 2003 at approximately 02:30 UTC or about 90 minutes after the lapse of the 48-hour deadline, at 05:33 local time, explosions were heard in Baghdad. Special operations commandos from the CIA's Special Activities Division from the Northern Iraq Liaison Element infiltrated throughout Iraq and called in the early air strikes. At 03:15 UTC, or 10:15 p.m. EST, George W. Bush announced that he had ordered an "attack of opportunity" against targets in Iraq. When this word was given, the troops on standby crossed the border into Iraq.

Before the invasion, many observers had expected a lengthy campaign of aerial bombing before any ground action, taking as examples the 1991 Persian Gulf War or the 2001 invasion of Afghanistan. In practice, U.S. plans envisioned simultaneous air and ground assaults to decapitate the Iraqi forces quickly (Shock and Awe), attempting to bypass Iraqi military units and cities in most cases. The assumption was that superior mobility and coordination of Coalition forces would allow them to attack the heart of the Iraqi command structure and destroy it in a short time, and that this would minimize civilian deaths and damage to infrastructure. It was expected that the elimination of the leadership would lead to the collapse of the Iraqi Forces and the government, and that much of the population would support the invaders once the government had been weakened. Occupation of cities and attacks on peripheral military units were viewed as undesirable distractions.

Following Turkey's decision to deny any official use of its territory, the Coalition was forced to modify the planned simultaneous attack from north and south. Special Operations forces from the CIA and US Army managed to build and lead the Kurdish Peshmerga into an effective force and assault for the North. The primary bases for the invasion were in Kuwait and other Persian Gulf nations. One result of this was that one of the divisions intended for the invasion was forced to relocate and was unable to take part in the invasion until well into the war. Many observers felt that the Coalition devoted sufficient numbers of troops to the invasion, but too many were withdrawn after it ended, and that the failure to occupy cities put them at a major disadvantage in achieving security and order throughout the country when local support failed to meet expectations.

The invasion was swift, leading to the collapse of the Iraqi government and the military of Iraq in about three weeks. The oil infrastructure of Iraq was rapidly seized and secured with limited damage in that time. Securing the oil infrastructure was considered of great importance. In the Persian Gulf War, while retreating from Kuwait, the Iraqi army had set many oil wells on fire, in an attempt to disguise troop movements and to distract Coalition forces. Before the 2003 invasion, Iraqi forces had mined some 400 oil wells around Basra and the Al-Faw peninsula with explosives. Coalition troops launched an air and amphibious assault on the Al-Faw peninsula during the closing hours of March 20 to secure the oil fields there; the amphibious assault was supported by warships of the Royal Navy, Polish Navy, and Royal Australian Navy.

The United States Marine Corps' 15th Marine Expeditionary Unit, attached to the British 3 Commando Brigade and the Polish Special Forces unit GROM attacked the port of Umm Qasr. There they met with heavy resistance by Iraqi troops. A total of 14 Coalition troops and 30-40 Iraqi troops were killed, and 450 Iraqis taken prisoner. The British Army's 16 Air Assault Brigade also secured the oil fields in southern Iraq in places like Rumaila while the Polish commandos captured offshore oil platforms near the port, preventing their destruction. Despite the rapid advance of the invasion forces, some 44 oil wells were destroyed and set ablaze by Iraqi explosives or by incidental fire. However, the wells were quickly capped and the fires put out, preventing the ecological damage and loss of oil production capacity that had occurred at the end of the Persian Gulf War.

In keeping with the rapid advance plan, the U.S. 3rd Infantry Division moved westward and then northward through the western desert toward Baghdad, while the 1st Marine Expeditionary Force moved along Highway 1 through the center of the country, and 1 (UK) Armoured Division moved northward through the eastern marshland.

During the first week of the war, Iraqi forces fired a Scud missile at the American Battlefield Update Assessment center in Camp Doha, Kuwait. The missile was intercepted and shot down by a Patriot missile seconds before hitting the complex. Subsequently, two A-10 Warthogs bombed the missile launcher.