War of the Buttons (1962)

Original title: La Guerre des boutons

For generations, two rival French villages, Longueverne and Velrans, have been at war. But this is no ordinary conflict, for the on-going hostilities are between two armies of young schoolboys. When he is beaten by his father for having lost his buttons, the leader of the Longueverne army, Lebrac, has an idea which will give his side the advantage: next time, he and his brave soldiers will go in battle without their clothes...