Answer:
Germany's use of the Blitzkrieg tactic and previously used Schlieffen Plan had knocked out France swiftly.
Hitler's policies were massive militerization, he had created a new airforce called the Luftwaffe that helped assault French ground forces. He also created new types of tanks such as the Panzer I & II which were used mainly as swift armored vehicles. France had focused its forces on the Maginot Line that bordered Germany but stopped at the Belgium border. The Ardennes Forest also was unprotected because the French thought it was naturally unpenetrable due to its mountainous and heavily forest areas. Hitler's Armies had taken a huge risk by sending most of their forces through Belgium to cut off British and French armies in Northern France. The French and British mindset thought this war was going to be a repeat of WW1 with trench warfare and long stalemates. They had heavily left their guard down and underestimated the sheer power of the German Army.