And they were not the only ones who underestimated the Russians...(from the link I referenced earlier)
It’s only with hindsight that we see the decision to invade the Soviet Union as the act of a madman. In fact, “smart” opinion at the time was exactly the opposite.
“The best opinion I can get,” wrote William F. Knox, secretary of the navy, to President Roosevelt on June 23, 1941, “is that it will take anywhere from six weeks to two months for Hitler to clean up on Russia.” While in Britain the War Office told the BBC that they should not give the impression that the Soviets could hold out for longer than six weeks. The prevailing informed wisdom was summed up by Hugh Dalton, a member of the British cabinet, who wrote in his diary on June 22, 1941, “I am mentally preparing myself for the headlong collapse of the Red Army and Air Force.”