Which historical factor most influenced the breakup of the Soviet Union in 1991

The collapse of the USSR was the process of systemic disintegration in the social structure, national economy, and political sphere of the Soviet Union, which led to the termination of its existence on December 26, 1991.  

The disintegration process began in the second half of the 1980s with the beginning of perestroika; manifested itself, in particular, in the desire of the Soviet republics for greater state and economic independence from the central government and ended with the signing of the Belovezhskaya agreements on December 8 and the Alma-Ata declaration on December 21, establishing a confederal union of former Soviet republics, the Commonwealth of Independent States and the adoption of the declaration on the termination of the existence of the USSR on December 26, 1991.

to unite muslims and hindus in resistance against british colonialism.