The post-Soviet states, also referred to as the former Soviet Union (FSU) or the former Soviet republics, are the independent sovereign states that emerged/re-emerged from the dissolution of the Soviet Union in 1991. Prior to their independence, they existed as Union Republics, which were the top-level constituents of the Soviet Union. There are 15 post-Soviet states in total: Armenia, Azerbaijan, Belarus, Estonia, Georgia, Kazakhstan, Kyrgyzstan, Latvia, Lithuania, Moldova, Russia, Tajikistan, Turkmenistan, Ukraine, and Uzbekistan. Each of these countries succeeded their respective Union Republics: the Armenian SSR, the Azerbaijan SSR, the Byelorussian SSR, the Estonian SSR, the Georgian SSR, the Kazakh SSR, the Kirghiz SSR, the Latvian SSR, the Lithuanian SSR, the Moldavian SSR, the Russian SFSR, the Tajik SSR, the Turkmen SSR, the Ukrainian SSR, and the Uzbek SSR. In Russia, the term "near abroad" is sometimes used to refer to the post-Soviet states other than Russia.

NATO is a military alliance of thirty-two European and North American countries that constitutes a system of collective defense. The process of joining the alliance is governed by Article 10 of the North Atlantic Treaty, which allows for the invitation of "other European States" only and by subsequent agreements. Countries wishing to join must meet certain requirements and complete a multi-step process involving political dialog and military integration. The accession process is overseen by the North Atlantic Council, NATO's governing body. NATO was formed in 1949 with twelve founding members and has added new members ten times. The first additions were Greece and Turkey in 1952. In May 1955, West Germany joined NATO, which was one of the conditions agreed to as part of the end of the country's occupation by France, the United Kingdom, and the United States, prompting the Soviet Union to form its own collective security alliance later that month. Following the end of the Franco regime, newly democratic Spain chose to join NATO in 1982.