How did the the soviet union grew to include fifteen republics.

Soviet Union, in full Union of Soviet Socialist Republics (U.S.S.R.), Russian Soyuz Sovetskikh Sotsialisticheskikh Respublik or Sovetsky Soyuz, former northern Eurasian empire (1917/22–1991) stretching from the Baltic and Black seas to the Pacific Ocean and, in its final years, consisting of 15 Soviet Socialist Republics (S.S.R.’s): Armenia, Azerbaijan, Belorussia (now Belarus), Estonia, Georgia, Kazakhstan, Kirgiziya (now Kyrgyzstan), Latvia, Lithuania, Moldavia (now Moldova), Russia, Tajikistan, Turkmenistan, Ukraine, and Uzbekistan. The capital was Moscow, then and now the capital of Russia

1. antarctica2. saudi arabia

I'm pretty sure it's c. extremely gradual but not 100% sure though.