The United States invaded Panama on December 20, 1989, codenamed Operation Just Cause. The U.S. stated the operation was "necessary to safeguard the lives of U.S. citizens in Panama, defend democracy and human rights, combat drug trafficking, and secure the neutrality of the Panama Canal as required by the Torrijos–Carter Treaties". The US reported 23 servicemen killed and 324 wounded, with the number of Panamanian soldiers killed estimated at around 450. The estimates for civilians killed in the conflict ranges from 200 to 4,000. The United Nations put the Panamanian civilian death toll at 500, Americas Watch estimated 300, the United States gave a figure of 202 civilians killed and former US attorney general Ramsey Clark estimated 4,000 deaths. It represented the largest United States military operation since the Vietnam War. The number of US civilians (and their dependents), who had worked for the Panama Canal Commission and the US military, and were killed by the Panamanian Defense Forces, has never been fully disclosed.