Warsaw Pact. Following Stalin's death in 1953, a period known as de-Stalinization occurred under the leadership of Nikita Khrushchev. The Soviets took an early lead in the Space Race with the first artificial satellite, the first human spaceflight, and the first probe to land on another planet (Venus). In the 1970s, there was a brief détente in the Soviet Union's relationship with the United States, but tensions resumed following the Soviet invasion of Afghanistan in 1979. In the mid-1980s, the last Soviet leader, Mikhail Gorbachev, sought to reform the country through his policies of glasnost and perestroika. In 1989, during the closing stages of the Cold War, various countries of the Warsaw Pact overthrew their Marxist–Leninist regimes, which was accompanied by the outbreak of strong nationalist and separatist movements across the entire Soviet Union. In 1991, Gorbachev initiated a national referendum—boycotted by the Soviet republics of Lithuania, Latvia, Estonia, Armenia, Georgia, and Moldova—that resulted in the majority of participating citizens voting in favour of preserving the country as a renewed federation. In August 1991, hardline members of the Communist Party staged a coup d'état against Gorbachev; the attempt failed, with Boris Yeltsin playing a high-profile role in facing down the unrest, and the Communist Party was subsequently banned. The Russian Federation became the Soviet Union's successor state, while all of the other republics emerged from the USSR's collapse as fully independent post-Soviet states. The Soviet Union produced many significant social and technological achievements and innovations. It had the world's second-largest economy, and the Soviet Armed Forces comprised the largest standing military in the world. An NPT-designated state, it possessed the largest arsenal of nuclear weapons in the world. It was a founding member of the United Nations as well as one of the five permanent members of the United Nations Security Council. Before the dissolution, the country had maintained its status as one of the world's two superpowers through its hegemony in Eastern Europe, military and economic strengths and scientific research. Etymology The word soviet is derived from the Russian word (), meaning 'council', 'assembly', 'advice', ultimately deriving from the proto-Slavic verbal stem of ('to inform'), related to Slavic ('news'), English wise, the root in ad-vis-or (which came to English through French), or the Dutch ('to know'; compare meaning 'science'). The word sovietnik means 'councillor'. Some organizations in Russian history were called council (). In the Russian Empire, the State