Fairly obviously, this film is The Battle of the Bulge (1965)
The incident in question here is the Malmady Massacre
Schumacher later takes control of a vital intersection of three roads connecting Ambleve, Malmedy and the Siegfried Line. He sabotages the road signs, and the rear echelon of Wolenski's troops take the wrong road to Malmedy; almost the entire unit is captured and massacred. Lt. Weaver manages to escape, but Duquesne is killed. US soldiers become suspicious when they witness Schumacher's "military police" lay explosives incorrectly on the Our bridge, and his masquerade is revealed, though too late to stop Hessler.
For further reading, the real background to the massacre is here