On June 11 1937, the NKVD (old style KGB) executed eight Soviet high ranking officers (one Red Army Marshal included). A military court found them guilty of anti-Soviet conspiracy.
In order to keep the purge productive and secret, most members of the court later died of pneumonia caused by cold bullets.
By the time this cadre rotation was over, Stalin removed 3 of 5 marshals, 13 of 15 army commanders, 8 of 9 admirals,50 of 57 army corps commanders, 154 out of 186 division commanders, 16 of 16 army commissars, and 25 of 28 army corps commissars. The chances of survival increased if the respective officer sported a huge mustache (see Marshal Semyon Mikhailovich Budyonny).