Many records for invasions were set during World War II, at the peak of second and third generation warfare. The vast numbers of the armies involved combined with innovative tactics and technology lent themselves to invasions on a scale that had not been seen before and have not been seen since.
The largest land invasion in history was 1941's Operation Barbarossa, in which 3,900,000 German troops blitzkrieged into the Soviet Union. Initially the Germans advanced with great ease and nearly captured Moscow, also laying siege to Leningrad, but soon found themselves fighting the harsh Russian winter as well as stiffer Soviet resistance, and their advance ground to a halt at Stalingrad in early 1943.
In the largest amphibious invasion in history, 156,215 Allied troops landed at Normandy to retake France from the occupying German forces. Though it was costly in terms of men and materials, the invasion advanced the Western Front and forced Germany to redirect its forces from the Russian and Italian fronts. In hindsight, the operation is also credited with defining the Western boundary of Soviet communism; had the Allies not advanced, it is conceivable that the Soviet Union would have controlled more of Europe than it eventually did.
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