"Katyusha" is a Soviet love song from the Second World War. The song is about a woman called Katyusha sitting by a river, singing about her lover who had gone to fight in the war. Even though it was created before the war had started, it exploded in popularity once the war had began, as many Soviet soldiers had to leave their loved ones behind to fight.
It was insanely popular in the Soviet Union, and retained it's popularity after the the dissolution, still being a well loved song in post-Soviet countries like Russia, Kazakhstan, Belarus and until recently, Ukraine.
The song was written by Mikhail Isakovsky and composed by famous Soviet composer Matvey Blanter,. It was released in 1938, 3 years before the Second World War began in the Soviet Union.