Gorbachev praised resistance to the war in the U.N. Security Council by Russia, France and Germany, telling reporters the three countries "showed great responsibility and were correct."
"International law was cast aside," the former Soviet leader said. "The country that made democracy its banner ignored the majority."
Gorbachev, calling for the Security Council to take control of postwar Iraq, accused Washington of "imperial behavior."
A November resolution required Iraq to cooperate with U.N. weapons inspectors or face "serious consequences," but did not specify the use of force.
The United States launched military action in March after abandoning efforts to secure a second Security Council resolution approving an attack. President Jacques Chirac said France would use its veto to block the resolution.
Gorbachev is currently chairman of the Gorbachev Foundation, a policy advocacy organization with a North American headquarters at Northeastern University in Boston.