American warships fired cruise missiles on targets in the capital Baghdad and Stealth fighters dropped bombs.
US President George W Bush appeared live on television saying that air attacks had started.
He said he was determined to "disarm Iraq and free its people".
And he warned the military attacks could go on for longer than some people expect.
'Holy war'
America says the attacks were aimed at senior people in the Iraqi leadership, including President Saddam Hussein.
Saddam Hussein appeared on Iraqi television a few hours after the American assault and said a "holy war" had begun.
Battle position
Some people are surprised at the time war started, as America chose to wait until daylight - attacks usually happen at night.
Around 150,000 combat troops are in battle position ready for ground attacks on Iraq, once the order has been given.
Iraq fired two missiles into the northern Kuwait desert on Thursday morning.
Troops and reporters have been ordered to put on gas masks and protective suits.
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