There was a civil war in 1918 because there was increasing opposition to the Bolsheviks. This opposition came from indigenous people, Czechs legions in Russia, and the West.
Local Russians opposed the Bolsheviks for a number of reasons which include, but the two most important are the loss of land, people and industry resulting from the treaty of Brest-Litovsk and the dishonoring of the new government. The treaty of Brest-Litovsk, signed march 3rd 1918 was meant to get Russia out of the Great War, and it obliged Russia to cede 32% of Russia's agricultural land, 34% of the population and 54% of Russia's industry. After seizing power, Lenin allowed elections to be held for the Assembly. The Bolsheviks won 170 seats while the social revolutionaries won more than twice as much, 370 seats. However, when the assembly was scheduled to start meeting, Lenin saw to it that the Red army had it closed, and "silenced" anyone who complained.
The Bolsheviks represented a minority of the Russian population that had managed to rise to power, however the majority of the Russian population was dissatisfied with the Bolsheviks, for some of the reasons listed above. The Social Revolutionaries were dissatisfied because they were denied political power, and together with the Mensheviks, Tsarists, Landlords, etc. they were all angry at the outcome of the treaty of Brest-Litovsk.
In May 1918 there was an uprising of the Czechoslovak brigade in Russia. The Czechoslovak brigade was the best organized military force in Russia, surviving the collapse of the imperial Russian army, which it was once a part of. The Czechs quarreled with the Russians because they wished to go to the French front to fight for an independent Czechoslovakia. Most important is what they did afterwards, which is that they took control of...