After it was established on most of the territory of the Russian Empire, the Soviet Union remained the world's largest country until it collapsed in 1991. It covered a large part of Eastern Europe while also spanning the entirety of the Caucasus, Central Asia, and Northern Asia. During this time, Islam was the country's second-largest religion; 90% of Muslims in the Soviet Union were adherents of Sunni Islam, with only around 10% adhering to Shia Islam. Excluding the Azerbaijan SSR, which had a Shia-majority population, all of the Muslim-majority Union Republics had Sunni-majority populations. In total, six Union Republics had Muslim-majority populations: the Azerbaijan SSR, the Kazakh SSR, the Kyrgyz SSR, the Tajik SSR, the Turkmen SSR, and the Uzbek SSR. There was also a large Muslim population across Volga–Ural and in the northern Caucasian regions of the Russian SFSR. Across Siberia, Muslims accounted for a significant proportion of the population, predominantly through the presence of Tatars. Following their seizure of power, the Bolsheviks wanted to include as much territory of the former Russian Empire as possible into the nascent Soviet Union. However, they experienced difficulties during their attempts to impose communism within regions that had been under traditionalist Islamic influence for centuries (i.e., onto the Turkic peoples). While the communist state actively pursued a policy of state atheism, Soviet authorities allowed limited religious activities to continue in the six Muslim-majority Union Republics. Mosques operated in most large cities within these territories, though their numbers decreased dramatically; there were 25,000 mosques across the Russian Empire at the time of the Bolshevik Revolution in 1917, and only 500 mosques across the Soviet Union in the 1970s. In 1989, as part of new Soviet policies that relaxed religious restrictions throughout the country, a number of new Muslim associations were formed and many of the mosques that had been closed by the government were reopened. The Soviet government also announced plans to permit the education of a limited number of imams in the cities of Ufa and Baku. In the years leading up to the dissolution of the Soviet Union, between 45 and 50 million Soviet citizens identified as Muslims, though only around 500 mosques were in operation across the country; Soviet law forbade all Islamic religious activities outside of mosques and madrasas. All Islamic religious facilities were supervised by four "Spiritual Directorates" established by the Soviet government in order to provide oversight.