WASHINGTON (CNN) -- President Bush announced Wednesday night he has ordered the coalition attack on Iraq to begin.

"American and coalition forces are in the early stages of military operations to disarm Iraq, to free its people and to defend the world from grave danger," Bush said.

He said the first strikes were against "selected targets of military importance," including what Pentagon officials said was a "decapitation attack" early Thursday morning to take out Iraqi President Saddam Hussein.

Administration sources said the decision to strike came after a nearly four-hour meeting in the Oval Office in which CIA Director George Tenet and Pentagon officials told Bush that they could lose the "target of opportunity" if they didn't act quick; Bush then gave the green light.

Whether the mission succeeded is not known. Pentagon officials said it is very difficult to successfully target a single person on the ground in such a bombing.

Two dozen cruise missiles were fired from U.S. warships in the Red Sea and Persian Gulf, military officials said. F117 stealth fighters, which carry two missiles apiece, also were involved in the attack.

Air raid sirens were heard in Baghdad at about 5:30 a.m. Thursday (9:30 p.m. Wednesday ET) about 90 minutes after the U.S. deadline for Saddam to step down or face a U.S.-led military attack.

In his four-minute announcement from the Oval Office, Bush said the military campaign, supported by 35 nations, would make efforts to spare Iraqi civilians. But he made it clear the U.S. military planned to use its full might in the war.

"This will not be a campaign of half measures, and we will accept no outcome except victory," he said.

The president's address came at 10:15 p.m., about two hours and 15 minutes after the expiration of the deadline.

The United States and Britain have massed nearly 300,000 troops in the Persian Gulf region.

Earlier, Adm. Timothy Keating, commander of the U.S. 5th Fleet, said allied forces were prepared to carry out an "unprecedented" campaign: "If we go, the plans we have are unlike anything anyone has ever seen before."