The war started because the US and UK governments believed Iraq had weapons of mass destruction.
These can be nuclear, biological and chemical weapons, and can kill many people.
Iraq's former leader, Saddam Hussein, said they didn't have any weapons like this.
But America and the UK didn't believe him. They also said Saddam was a cruel leader who treated his people badly, so they sent in troops to disarm Iraq by force.
So far, no weapons of mass destruction have been found in Iraq.