The beginning of the Second World War was extremely unfortunate for the Red Army. In the first 1.5–2 years of the war, the Red Army suffered a number of heavy defeats. Why did this happen? Among several reasons, the main one is the leadership of the USSR ignoring the achievements of military science. In the early 1930s. In the USSR, a new modern theory of warfare was developed - the theory of deep operations. It is based on the creation of tank formations, which, in cooperation with aviation and other branches of the military, delivered a powerful blow to break through the entire depth of the enemy’s defense, which was not possible during the 1st World War. A group of military men turned to People's Commissar Voroshilov with a proposal to accelerate the arming of the Red Army with tanks by reducing the cavalry. But Voroshilov was against this. After which they began to discuss the issue of replacing the People's Commissar. Not wanting to leave office, the “Military Conspiracy” case led by Marshal Tukhachevsky was fabricated. The trial that took place sentenced them to death. After the trial, this theory was banned.