1972 in the Vietnam War the number of air raids against North Vietnam during all of 1971. Prime Minister Lon Nol declared President of Cambodia. Unlike the former Cambodian leader, Norodom Sihanouk, Lon Nol was a U.S. ally. The U.S. 101st Airborne Division left South Vietnam, the last U.S. ground combat division to be withdrawn from South Vietnam. The Khmer Rouge bombarded with artillery Phnom Penh, the capital city of Cambodia, killing more than 100 civilians. This was the heaviest attack on Phnom Penh since the Cambodian Civil War began in 1970. The United States boycotted peace negotiations in Paris with the North Vietnamese, citing
U.S.–German Peace Treaty (1921) to consent to ratification of the treaty, due in large measure to its objections to U.S. participation in the League of Nations. As a result, the two governments started negotiations for a bilateral peace treaty not connected to the League of Nations. On July 2, 1921, U.S. President Warren G. Harding signed the Knox–Porter Resolution, which had been passed by the U.S. Congress and ended the state of war between the U.S. and Germany, Austria and Hungary, further setting the stage for bilateral peace treaties. The treaty between the U.S. and Germany, formally titled the "Treaty between the United States