Answer
Mikhail Frunze sided with Vladimir Lenin following the ideological split in the Russian Social Democratic Labour Party (RSDLP). This split occurred in 1903 and resulted in the formation of two separate factions: the Bolsheviks, led by Lenin, and the Mensheviks, led by Julius Martov. Frunze joined the Bolsheviks and quickly rose through the ranks, becoming one of Lenin's most trusted lieutenants.