The Fifth Ambush (1968)

A partisan battalion falls into enemy trap more than once, which makes them suspect it's about treason. Their commissar and commander move the unit in a remote settlement in order to discover the enemy agent. This draws a lot of suspicion and distrust within the unit, and finally it was the head of the battalion who was charged as guilty, and executed. However, was he the real intruder? The commissar and the commander take the battalion to a new battle without definitive knowledge of the whole present situation.