<human>:  Why did the US invade Iraq in 2003?
<bot>: The US invaded Iraq in 2003 because Saddam Hussein had weapons of mass destruction. He had been trying to build nuclear weapons since the 1980s, and the US believed that he was trying to build chemical weapons as well. After the first Gulf War in 1991, the UN imposed sanctions on Iraq, but Saddam Hussein refused to comply. In 2002, the US asked the UN Security Council to pass a resolution that would give the US permission to invade Iraq if Saddam Hussein did not comply with the UN sanctions. Saddam Hussein refused to comply, so the US invaded Iraq.