Pugwash's first fifteen years coincided with the Berlin Crisis, the Cuban Missile Crisis, the Warsaw Pact invasion of Czechoslovakia, and the Vietnam War. Pugwash played a useful role in opening communication channels during a time of otherwise-strained official and unofficial relations. It provided background work to the Partial Test Ban Treaty (1963), the Non-Proliferation Treaty (1968), the Anti-Ballistic Missile Treaty (1972), the Biological Weapons Convention (1972), and the Chemical Weapons Convention (1993). Former US Secretary of Defense Robert McNamara has credited a backchannel Pugwash initiative (code named PENNSYLVANIA) with laying the groundwork for the negotiations that ended the Vietnam War. Mikhail Gorbachev admitted the influence of the organisation on him when he was leader of the Soviet Union. In addition, Pugwash has been credited with being a groundbreaking and innovative "transnational" organization and a leading example of the effectiveness of Track II diplomacy.