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He was elected Chairman of Petrograd Soviets in 1917, which gave him immense practical power over the city (control of bridges, railways etc.), which was a valuable assets to Bolsheviks. He also used his position as Chairman to claim that the Bolsheviks were seizing power in the name of the Soviets, and hence workers accepted that Bolsheviks conducted the revolution. It was not until Lenin closed down the new parliament that workers realized that they had been fooled.
What role did Trotsky play in the Revolution in 1917?