I've always been fascinated by the Russo-German war of 1941-1945, that began with the launch of Operation Barbarossa and ended with the Soviet Army overrunning Berlin.
The initial German attack with a army of over 3 million combat troops, the massive Soviet losses, in the millions, in the initial stages; the titanic struggle at Stalingrad in 1942, the largest tank battle in history at Kursk in 1943, and on and on. The scale dwarfs all other warfare humans have ever taken part in.
One example: We've just celebrated the 75th anniversary of D-Day, a massive military operation by Western Front standards in World War 2. But the same month - June 1944 - the Soviets launched Operation Bagration, an attack using more than two million soldiers, aimed at wrecking the German Army Group Center and pushing the Eastern Front out of Russia and into Poland. It was a much bigger military undertaking than D-Day, but few in the US have heard of it.