France fought the Germans for four long years during World War I, and while they suffered heavily in casualties and economic losses, they were never defeated.  They didn't count on the Blitzkrieg tactics of World War II Germany.  By May of 1940 the German Army had perfected the new style of fighting, and used it with brilliant effectiveness to conquer France in only six weeks.

Britain and the world were stunned, and the British Army barely escaped at Dunkirk with their lives (and none of their weapons).  This would effectively lock up continental Europe from Allied forces for the next four years, allow Hitler to attack England from new air bases in France, and to shift his armies eastward to attack Russia.