"It is interesting to see how Western propaganda, via Robert Conquest, has lied about the purges of the Red Army. Conquest says in his book The Great Terror that in 1937 there were 70,000 officers and political commissars in the Red Army and that 50% of them (i.e., 15,000 officers and 20,000 commissars) were arrested by the political police and were either executed or imprisoned for life in labour camps. In this allegation of Conquest’s, as in his whole book, there is not one word of truth. The historian Roger Reese, in his work The Red Army and the Great Purges, gives the facts which show the real significance of the 1937-38 purges for the army. The number of people in the leadership of the Red Army and air force, i.e., officers and political commissars, was 144,300 in 1937, increasing to 282,300 by 1939. During the 1937-38 purges, 34,300 officers and political commissars were expelled for political reasons. By May 1940, however, 11,596 had already been rehabilitated and restored to their posts. This meant that during the 1937-38 purges, 22,705 officers and political commissars were dismissed (close to 13,000 army officers, 4,700 air force officers and 5,000 political commissars), which amounts to 7.7% of all officers and commissars - not 50% as Conquest alleges. Of this 7.7%, some were convicted as traitors, but the great majority of them, it would appear from historical material available, simply returned to civilian life.” (source)
The percentage of military officer personnel purged during this period is dramatically exaggerated, and a number of those who were purged, such as Tukhachevksy and Tokaev, were guilty of plotting military coups. In light of that, I don’t see how the purges of the military could’ve dramatically impacted the USSR’s war effort in any serious way unless we assume that such men would’ve helped the USSR’s war effort.