Dunkerque - 1940 The Campaign for France, 20 - 31 May, 1940. From 20 May until the panzer withdrawal and reassembly on 31 May for "Fall Rot," the second phase of the campaign, was the most crucial part of the battle. The elite northern armies, relatively untouched until then, were going to try to prevent the Germans panzers from cutting them off, or at worst, escape to fight another day.