Battle of the Bulge
A battle in 1944; pretty much the last chance the Germans had to break through to the Channel ports and stop the war on the Western Front.
This Wiki page will tell you almost everything you need to know.
During the battle, due to rumours about third columnists, GI's started quizzing each other about baseball scores in an attempt identify the krauts in their midst. This led to GI's who didn't like baseball being locked up for being spies.