The previously mentioned break-through at Sedan and the fact that fast-moving German units were now racing towards the English Channel without any major force available to stop them forced the French and British forces in Belgium to reconsider their situation.
The allied forces which had retreated to the Dyle line had been able to slow the Wehrmacht‘s advance – cities such as Antwerp, Leuven and Brussels itself were still under Belgian control. However, the danger of being encircled by the Wehrmacht called for a change – the French 1st Army withdrew from the Antwerp area, the British Expeditionary Force also started to fall back.
Outnumbered, the Belgian forces abandoned Brussels, the British had abandoned Leuven. Both cities fell to the Germans. The situation map of May 17, 1940, shows the “jump” westward.