RA

The abbreviation stands for "Red Army". In USSR, it was short for "Workers' and Peasants' Red Army".
The term comes from Russian civil war, 1917-1922, in which the victorious Reds (bolsheviki/ communists) fought against Whites (monarchists-liberals) and Greens (anarchists). Red is a well-known political symbol of communism.
Today, most commonly, it refers for the land-based armed forces; may or may not include aviation; excludes navy and missile troops.

On 16th of April 1945, The Red Army began the Berlin Strategic Offensive Operation, which resulted in the unconditional surrender of German troops on the night of 8 to 9 May 1945.