Trotsky fought in Russia's Revolution in 1917 with Lenin (the founder of the Russion Communist Party). He was a commissioner in war and defeated forces that went against the Bolshevik control. The government got more and more powerful and so did Stalin (the dictator of the USSR). He eventually lost power and was exiled and later killed.
What he mainly did
1. He fought in the big Russian Revolution.
2. Deleted the enemies of the Bolshevik control.