They used Maginot Line. The Maginot Line was built to fulfill several purposes: To prevent a German surprise attack. ... To push Germany into an effort to circumvent via Switzerland or Belgium, and allow France to fight the next war off French soil to avoid a repeat of 1914–1918. To be used as a basis for a counter-offensive.
Hitler ordered a conquest of the Low Countries to be executed at the shortest possible notice to forestall the French and prevent Allied air power from threatening the vital German Ruhr Area. It would also provide the basis for a long-term air and sea campaign against Britain
