"Iraq's chemical, biological, and nuclear weapons programs, together with Iraqi long-range missile development and support for terrorism, are the primary justifications put forward for military action. On March 17, 2003 President Bush issued an ultimatum demanding that Saddam Hussein and his sons depart from Iraq within 48 hours. On March 19, offensive operations began with air strikes against Iraqi leadership positions. Offensive operations combine an air offensive with advancing elements of four armored, mechanized, airborne, and Marine divisions. It is a smaller force than the 1991 Desert Storm operation, reflecting an assessment that Iraqi armed forces are neither as numerous nor as capable as they were ten years ago, and that U.S. forces are significantly more capable. This size force was generally considered to offer greater certainty than lighter ground force options for a rapid victory against significant Iraqi resistance. As the U.S. ground offensive approaches Baghdad, DOD civilian leadership has come under criticism for not permitting the deployment of sufficient U.S. ground forces to maintain the offensive, protect lines of supply, and secure rear areas where Iraqi resistance continues. U.S. Central Command maintains that offensive operations are 'on track', but have begun to acknowledge that Iraqi resistance is stronger than anticipated."
CRS Report for Congress, RL31701
U.S. Dept. of State, Foreign Press Centers: http://www.fpc.state.gov/