The way the Stalinist regime treated its own people demoralised not just the military but the common folk as well. The morale of the average Russian soldier in the early years of WWII was quite low and is one reason why millions were quick to surrender en masse in 1941. In some areas it wasn't only the Ukrainians, Balts and Estonians welcoming the Germans with showers of flowers, but ethnic Russians as well.
Many often forget that the Great Purge wasn't just about purging military and government officials; it also affected entire groups of people on the civilian side. However I can't elaborate on that because I'd be going into a topic that we're not allowed to talk about on these boards and I really don't want to get banned. But point being, the purges only served to demoralise the people. Hardly an up in national unity.
National unity is not only loyalty of population to regime ,it is also political unity. Purges demoralized officer corps,party officials and part of urban population ,but also eliminated any political opposition and Stalin was completely free in his decisions. I doubt that without purges Stalin could go for Molotov-Ribentrop pact,for an example. Also,loyalty of rural population was not changed much since purges did not influence it that much.
So the approximation that purges raised national unity while damaging organization and recovery rate seems pretty decent to me.
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