Its the start of World War 2, September 3, 1939 the United Kingdom and France declares war on Germany in response to its invasion of Poland. This prompted France to build a long installation of fortresses along the France border that would be called the Maginot Line named after the French Minister of War (Andre Maginot).
It was praised by French military experts to be an impenetrable defense deterring German surprise invasions. The main purpose of the Maginot line was to keep the war of French soil and keep from a repeat of the first world war. However one flaw that they overlooked is the weakened defenses at the Ardennes Forest. They believed that the terrain was virtually impassable and even if Germany tried they would be stuck and give enough time for them to send reinforcements and mount a counter-offensive.
The bulk of the fortifications were along the German border where they believed Germany would launch their offensive and would use Belgium as a counteroffensive in the wake of the invasion. However, this false sense of security would be their downfall as Germany developed a new way of fighting which would be the bulk of their strategy of World War 2. Blitzkrieg. Which completely broke the doctorate of world war one that was based on stationary and stalemated battles. It consisted of fast, powerful attacks that was aimed at confusing enemy lines and make it difficult to respond the constantly moving front.
As shown here a classic Blitzkrieg would mostly consist of a column light panzers and mechanised infantry.
This can be all shown in the Manstein Plan (Germanys plan to penetrate the Maginot Line)
5 panzer divisions would go through the Ardennes and 3 panzer divisions would head to the southern flank in the Sedans. (To put into context a panzer division would consist of about 11,792 personnel). The French caught by surprised by the large force were forced to retreat. To slow the advance the Allies sent bombers to northern Belgium. The plan had gone ahead of schedule and the generals were ordered to halt but disobeyed and went on to the English Channel where the French were scrambling to bring in reinforcements as the German front was moving so fast that when reinforcements arrived they would be there already forcing them to fall back.
The aftermath of the attack would leave British and French forces surrounded.
In the Battle of Dunkirk, British forces were evacuated and although the French Army fought on, German troops entered Paris on 14 June. The French government was forced to negotiate an armistice at Compiègne on 22 June.