At the beginning of 1918, the Soviet government abolished the old imperial army, dating back to Tsardom, and formed the Red Army in its place, first from volunteers, later on the basis of conscription. Some ex-officers were recruited as trainers, but in reality the command was often in the hands of the revolutionary commissars. Lev Trotski made his mark as the organizer of the red army.

When the enemy was approaching Moscow, which the Soviet government had made the capital instead of the threatened St Petersburg, from all directions, Lenin ordered the overthrown Tsar Nicholas and his family to be executed in Jekaterinburg. The bolsheviks resorted to red terrorism, and their adversaries to white terrorism. The Civil War was to be cruel and fatal for many. Approximately one million men were killed and six and a half million wounded in the battles of the Civil War during the years 1918-1922.

At the beginning of the war in 1918, the influence of the German occupation army regulated the events on the western border areas of the Russian Empire, but when the Germans were forced to admit defeat in the World War in November 1918, they withdrew their troops also from the territories of the former Russian Empire. The Baltic countries struggling for independence were liberated, but soon became targets of Soviet attacks.

In Poland, Jozef Pilsudski’s government took power with the aim of reaching the old frontiers of 1772. This plan met with success as regards areas under German and Austrian control, but in the east, owing to the disputes between Poland and Soviet Russia, an undeclared war broke out (January - November 1919) after Russia had occupied Vilna, the bone of contention between Poland and Lithuania. The Poles were advancing towards the east.

In Ukraine, the socialist leader Simon Petljura overthrew the government of general Pavel Skoropadski after the departure of the Germans. The Ukrainian troops took possession of Galicia and established the republic of West-Ukraine. Only half a year later, in May 1919, they were, however, forced to yield the area to Poland. The Bolsheviks in their turn occupied Kiev on 3 February, 1919, and Odessa, which the French had kept in their possession for half a year, on 8 April, 1919.

The major battles of the Civil War were fought in southern Russia and Ukraine. In January 1919, general Anton Denikin drove the Bolsheviks out of Caucasia. He became Commander-in-Chief of the whites, after Kaledin had died in February and Kornilov in April, 1919. Denikin was also backed by P.N. Krasnov, the ataman of the cossacks. In May 1919, the white Russian troops attacked in the north, in the Baltic area in the north-west (general Nikolai Judenits), in the south-west (Denikin), in Caucasia in the south (general Pjotr Wrangel) and in the east.

In Siberia, the white Admiral Aleksandr Koltshak had been the "highest Russian authority" since November 1918, but his troops were forced to gradually retreat as early as the beginning of 1919, despite English, French and Japanese support. Since June 1918, a legion consisting of approximately 100,000 Czech war prisoners had been active in Siberia. Their purpose was to secure a safe journey back home by way of Vladivostok. The legion had seized the control of the Siberian railway, and gave their support to Koltshak.

The governments of the border powers (Finland, Estonia, Latvia, Lithuania, Poland, Ukraine etc.), which had recently gained independence, would have been natural allies of the white Russians. But as the white Russians were still aiming at maintaining the frontiers of the old Russian Empire, the governments of these countries did not want to get involved in battles outside their own borders. Consequently, Judenitsh, among others, could look for no support from Finland, and the white generals were incapable of coordinating their offensives. Their cooperation failed, whereas the Soviet troops could be rapidly transferred from one front to another.

After Koltshak had been forced to retreat from the Ural in early 1919, the reds attacked towards the Black Sea in October. At that point, Judenitsh with his army attacked from Estonia to the outskirts of St Petersburg, but Trotski managed to drive the whites back to Estonia. At the same time, the whites in the south withdrew to the coastal areas of the Black Sea. In the direction of Siberia, Koltshak lost control of the situation in late 1919.

In December 1919, the leaders of the Western Allies defined the so-called Curzon line as the eastern border of Poland, approximately at the level of the River Bug. This did not satisfy Poland, and she commenced war to defend her old areas. The troops of Pilsudski, supported by the Ukrainians of Petljura, occupied Kiev in May 1920, but as early as June and July they were forced to retreat as far as Warsaw under the pressure of the Soviet troops commanded by M.N. Tuhatsevski. The maintenance and supply system of the Soviet troops had, however, met with difficulties during the large operations, and Pilsudski managed to break their resistance in August 1920, and force them to retire as far as the Grodnon-Brest Litovsk-Wlodawan line. In September and October, the Poles staged a further offensive in front of Minsk. The Armistice was concluded in October1920, and the Riga Peace Treaty in March 1921. Russia consented to the territorial claims of Poland. Ukraine joined the Soviet states late in 1920. Hostilities began between Poland and Lithuania, when Poland took possession of the disputed Vilna in October 1920. This war continued inactively until December 1927.

As there seemed to be an imminent danger that Japan would expand to eastern Siberia, the United States sent troops to Vladivostok in August 1918, "to protect the departure of the Czechs." The Czechs were now finally able to leave after fighting against both red and white Russians. The U.S. troops kept guard over the Siberian railway from Baikal to Vladivostok, but did not participate in the battles against the Bolsheviks.