The Soviet Union's Red Air Force quickly recognized the value of ground-support aircraft. As early as the Battles of Khalkhyn Gol in 1939, Soviet aircraft had the task of disrupting enemy ground operations. This use increased markedly after the June 1941 Axis invasion of the Soviet Union. Purpose-built aircraft such as the Ilyushin Il-2 "Sturmovik" proved highly effective in blunting the activity of the "Panzers". Joseph Stalin paid the Il-2 a great tribute in his own inimitable manner: when a particular production factory fell behind on its deliveries, Stalin sent the following cable to the factory manager: "They are as essential to the Red Army as air and bread".