France (and Britain) were prepared for a German invasion through Belgium as in WWI - in fact, the entire French strategy was built around the Germans adopting just such a plan. The French had built the Maginot line to force the Germans into Belgium, where the best French mobile troops would meet them and defeat them. As Belgium was not part of the Allies until the German invasion, the French and British could not dig in within Belgian territory. Instead, they dug in along the Belgian border, in what British troops called the 'Gort Line' after the commander of the British Expeditionary Force (BEF). Once the invasion began, these troops were to move forwards to take up defensive positions along either the Escaut (closer to France) or Dyle (closer to Germany) rivers - with plans made for the defence of both lines, and named after the respective rivers.

Once Fall Gelb, the German invasion of France, began, the Dyle Plan was put into effect. The BEF and the French 1st Army Group moved into Belgium and took up positions along the Dyle, linking up with the Belgian Army and preparing to fight the German advance. However, this did not come in this sector. Instead, it came through the Ardennes, a wooded, hilly region on the Franco-Belgian-Luxembourgian border. This had few roads running through it - meaning that the French thought it an unlikely area for an advance to occur, as the spearhead could not be effectively supplied and it was easy to defend. The Ardennes was protected by the French 2nd Army, a weak formation composed heavily of reservists. Unable to hold against the German attack, this was forced to withdraw. The German forces pushing through the gap were able to push forwards to the Channel before the British and French forces in Belgium could redeploy to cut them off. As a result, the Allied forces in Belgium and Northern France were cut off, and had to be evacuated through Dunkirk.