With support from the US!
"The provision of chemical precursors from United States companies to Iraq was enabled by a Ronald Reagan administration policy that removed Iraq from the State Department's list of State Sponsors of Terrorism."
U.N. weapons inspector Hans Blix faults Bush Administration for lack of "critical thinking" in Iraq
"Washington, D.C., 25 February 2003 - The National Security Archive at George Washington University today published on the Web a series of declassified U.S. documents detailing the U.S. embrace of Saddam Hussein in the early 1980's, including the renewal of diplomatic relations that had been suspended since 1967. The documents show that during this period of renewed U.S. support for Saddam, he had invaded his neighbor (Iran), had long-range nuclear aspirations that would "probably" include "an eventual nuclear weapon capability," harbored known terrorists in Baghdad, abused the human rights of his citizens, and possessed and used chemical weapons on Iranians and his own people. The U.S. response was to renew ties, to provide intelligence and aid to ensure Iraq would not be defeated by Iran, and to send a high-level presidential envoy named Donald Rumsfeld to shake hands with Saddam (20 December 1983)."
Shaking Hands with Saddam Hussein - Press Release