<human>:  Why did the United States invade Iraq in 2003?
<bot>: The United States invaded Iraq in 2003 because Saddam Hussein had weapons of mass destruction. After the first Gulf War in 1991, the UN imposed sanctions on Iraq because they believed that Iraq had violated the terms of the ceasefire. Iraq had a history of using chemical weapons against their own people and against Iran. In 1998, Iraq was found to be in violation of the terms of the ceasefire, and in 2002, the UN Security Council voted to authorize military action against Iraq.