Revisionists are fond of pointing out that, without the Red Army, the western allies would never have defeated Germany in 1945. The theory overlooks an equally sobering fact: without the Red Army, Germany probably wouldn't have gone to war in the first place, thus sparing the world some 60 million early deaths and thrice that number of wounded, raped, and exiled.
In The Devil's Alliance, the British scholar and journalist Roger Moorhouse examines the August 1939 pact between Adolf Hitler and Joseph Stalin that divided Europe between them. Within a month, their armies had invaded and occupied Poland, and within a year (aided by Italy) they'd moved into every continental nation except Sweden, Switzerland, Portugal, and Spain--and those were half-hearted allies. Would any of this been possible without the Hitler-Stalin pact? It's very unlikely.
Nor was that the limit of their cooperation. Russian oil helped lubricate the German panzers as they crashed through the Ardennes Forest and routed the British and French defenders (though Mr. Moorhouse argues that food, not oil and raw materials, were Germany's major benefit from the pact). In turn, the Russians received German machine tools, locomotives, and weapons technology that helped the Red Army turn back the German onslaught after June 1942, when the partners finally found themselves at war.
The Second World War in Europe lasted 69 months. For 22 of them -- nearly a third of the war! -- Russia was on the wrong side, and arguably the war would never have happened if it hadn't been so eager to grab half of Poland in September 1939. Available on Amazon
in hardcover and Kindle editions.