For the benefit of others, here's what I found at Great Purges:
Stalin now decided to purge the Red Army. Some historians believe that Stalin was telling the truth when he claimed that he had evidence that the army was planning a military coup at this time. Leopold Trepper, head of the Soviet spy ring in Germany, believed that the evidence was planted by a double agent who worked for both Stalin and Hitler. Trepper's theory is that the "chiefs of Nazi counter-espionage" led by Reinhard Heydrich, took "advantage of the paranoia raging in the Soviet Union," by supplying information that led to Stalin executing his top military leaders.
In June, 1937, Mikhail Tukhachevsky and seven other top Red Army commanders were charged with conspiracy with Germany. All eight were convicted and executed. All told, 30,000 members of the armed forces were executed. This included fifty per cent of all army officers.