Combat MP directing supply traffic of the Red Ball Express.
Picture from the National Archives.

These guys also had to be on the lookout for a German Officer named Skorzeny, who had formed a unit of infiltrators to get behind Allied lines.
They spoke English, wore American uniforms and drove captured American vehicles. Some of them had grown up or attended school in the US and knew the slang.

Call challenges and countersigns were beaten out of captive prisoners, so the troops began asking about current events that the Germans wouldn't likely know, like who was pitching for the Yankees.

The Battle of the Bulge was rupturing the German defenses and Skorzeny's mission was to gain knowledge of Allied capabilities and be as disruptive as possible.
The steady stream of men and material up the Normandy beaches made it clear to Skorzeny that battles may be won, but the war was lost.