In the summer of 1939, all three brigades of the 20th Tank Corps were detached from the corps and sent into combat during the Battles of Khalkhin Gol. The 15th and 25th Tank Corps fought in the Soviet invasion of Poland in September 1939. As a result of the Soviet assessment of the tank corps as being unwieldy and difficult to control, shown by repeated traffic jams caused by the tank corps in Poland, the Main Military Council ordered their disbandment on 21 November, replacing the tank corps with 15 motorized divisions, each with two motorized rifle regiments, an artillery regiment, and a tank regiment. The tank corps were not actually disbanded until January 1940, by which time the 10th Tank Corps had seen brief service in the Winter War in December 1939.