Договор об образовании СССР
Constitutionally, the USSR was a federation of constituent Union Republics, which were either unitary states, such as Ukraine or Byelorussia (SSRs), or federations, such as Russia or Transcaucasia (SFSRs), all four being the founding republics who signed the Treaty on the Creation of the USSR in December 1922. In 1924, during the national delimitation in Central Asia, Uzbekistan and Turkmenistan were formed from parts of Russia's Turkestan ASSR and two Soviet dependencies, the Khorezm and Bukharan SSRs. In 1929, Tajikistan was split off from the Uzbekistan SSR. With the constitution of 1936, the Transcaucasian SFSR was dissolved, resulting in its constituent republics of Armenia, Georgia and Azerbaijan being elevated to Union Republics, while Kazakhstan and Kirghizia were split off from Russian SFSR, resulting in the same status. In August 1940, Moldavia was formed from parts of Ukraine and Bessarabia and northern Bukovina. Estonia, Latvia and Lithuania (SSRs) were also admitted into the union which was not recognized by most of the international community and was considered an illegal occupation. Karelia was split off from Russia as a Union Republic in March 1940 and was reabsorbed in 1956. Between July 1956 and September 1991, there were 15 union republics (see map below).