Vācietis started his military career in Imperial Russia in 1891. During World War I, he commanded the 5th Latvian Zemgale Rifle Regiment (eventually with the rank of colonel), which supported Lenin's Bolsheviks after the October Revolution and the ensuing Russian Civil War. From April 1918, Vācietis was the commander of the Red Latvian Riflemen division. From July to September 1918, Vācietis commanded the Eastern Front. Vācietis was the first commander-in-chief of the Red Army (RKKA), a member of the Revolutionary Military Council (2 September 1918 – 3 July 1919), forming a productive working relationship with Leon Trotsky, the Commissar for Military and Naval Affairs. On 3 July 1919, Sergey Kamenev replaced Vācietis as commander-in-chief of the Red Army, following certain discontent amongst some Bolsheviks over Red Army leadership in the Russian Civil War. On 8 July 1919, Vācietis was arrested under the accusation of membership in a counter-revolutionary reactionary White Guardist organization, but was soon released when the charge was proved false.
In 1922, Vācietis became a professor of the RKKA Military Academy (future Frunze Military Academy). During this time, he wrote several books, most notable among them being 'Latvian Riflemen's Historical Importance'. In 1935, Vācietis was assigned a personal rank of Comandarm, 2nd Rank.
On 29 November 1937, during the height of Stalin's Great Purge, Vācietis was arrested as a member of the alleged "Latvian Fascist Organization within the RKKA", and was executed on 28 July 1938. Vācietis was rehabilitated in 1957.