Hitler in Paris
World War I and its devastation traumatized France and the French. So they devised a plan to avoid a repetition: the Maginot Line. It would secure the Franco-German border to the south, while the mobile French army was stationed in the north, to advance into Belgium soon as the Germans attacked. That way the war would be fought as far forward and outside of France as possible.
The amassed enough mobile forces in the north to keep the Germans from bursting into France via that route, as they had done in WWI. However, they ignored a stretch of wooded terrain, the Ardennes Forrest, which they deemed impassable for tanks. The Germans figured the Ardennes was actually passable, so that was where they massed the bulk of their armor.
When the Germans burst through the Ardennes and raced to the English Channel to sever France’s armies in the north from the rest of the country, the French were wrong footed: their mobile forces were advancing into Belgium, and couldn’t be turned around in time to stop the Germans pouring out of the Ardennes, and they lacked adequate reserves to plug the widening gap.
Collapse quickly followed, and the same country that two decades earlier had fought the Germans for four bloody years and emerged victorious in WWI, capitulated and signed a humiliating surrender after just 40 days’ fighting in WWII. Hitler celebrated the victory by touring Paris, and his photos posing in front of the Eiffel Tower became emblematic of the French debacle.