I think Trotsky assured the Bolsheviks of a victory in the civil war. This is because Trotsky was an efficient leader who provided inspirational leadership to the Red Guards. He led the troops efficiently using his special armoured train, moving from one front to another quickly. He gave the troops encouragement when needed. He introduced conscription. Men aged 14-40 were made to join the army. This greatly expanded the army, which grew to 5 million men in 3 years under this scheme. Also he captured officers from the Whites and made them command his troops, shooting their families if they disobeyed. He assigned commissars to enforce discipline and boost morale in his troops. He meted out severe punishments to ensure the efficiency of his forces. For example, generals were given the death penalty for unauthorized retreats. Trotsky also ordered people to put up posters all around Russia to whip up support for the war.
However, there are other factors which brought victory to the Bolsheviks too. For example, political factors. Lenin, the political leader of Russia at that time, was an efficient leader. He knew that there would be people who would oppose Bolshevik rule and therefore pulled Russia out of WWI. This allowed the Bolsheviks to concentrate on fighting one war.
Support from the people cannot be ignored too. The people of Russia knew that if the whites won the civil war and restored monarchy, the land reforms would reverse. The peasants would no longer be able to own land. Also, the Reds portrayed themselves to be patriotic defenders of Russia, since they were fighting against the whites, who had foreign aid.