When Stalin offered to provide the ground force for an invasion of Nazi Germany, if the UK would give air support - pre Molotov-Ribbentrop pact - the reason the UK refused is supposedly because their assessment of Soviet military and industry was that they would be steamrolled by the Germans in such an action. Which to be clear isn’t true, given that the Soviet military was designed for offensive warfare (one of the reasons they didn’t perform very well at the start of Barbarossa) and the Nazi’s military was far weaker at this point, when they were still in the process of rearmament, compared to what it would eventually come to be.