How did Germany quickly defeat France in the Franco-Prussian War and World War II, but not in World War I?

A gap opened. Soldiers flew into that gap. Turns out Paris wasn't undefended, in fact the precise opposite; the French had been masterfully funneling soldiers into a new 6th Army into Paris from less necessary fronts. Really this is the war where French logistics outmatched German logistics; the Germans went into the Marne at 50-60% strength and the French managed to siphon every last troop they could from across the entire front to form an entirely new army of over a hundred thousand men. The finagling was insane to say the least. So when that gap opened over a hundred thousand Frenchmen along with about a hundred thousand British from the BEF swarmed into it and nearly encircled both of the strongest 'hammers' of the German army, the First and Second Army's.

So then everything collapsed on them. Turns out the French blew up all their railways as they withdrew and the Germans couldn't bring their enormous heavy weapons to the front as fast as they were advancing. Whereas the Germans had a 2:1 artillery advantage, with superior heavy artillery at that, in early July when the Battle of the Marne occurred in early September that advantage would exactly flip to the French having the 2:1 advantage. Turns out flat trajectory 75mm guns that can be pushed by two guys is much more feasible in a mobile campaign and they absolutely annihilated the Germans whose 77mm light guns could only fire 1 shot for every French 3. Twice the guns, thrice the firepower. Yeah.

The Germans would retreat and realize they could not win in the West just yet. The British were beginning to swarm more men in but really the French held the reigns until 1916. The Germans, having Russia begin to turn into a juggernaut in the East, would retreat to the most defensible positions in France/Belgium and dig in to basically hold off the French and British while they dealt with the Russians hopefully 3 on 1 (German/Austro-Hungarian/Ottoman alliance) while the French and British were happy as attrition was in their favor ultimately. From here on out it's history.


World War II

This discussion is slightly out of my wheelhouse. This post, however, by one of our former users is the most comprehensive overviews of the Fall of France and the reasoning why it was so sudden The TL;DR is that they would this time be massively outnumbered by the Germans; 76 to 135 divisions. By shortening the front in Belgium with the Maginot Line along with immediate British support the hope was to hold off the Germans as much as possible. The post paints the picture really well in the desperation of it;

The other consequence of France's manpower problems was that they were forced into using sub par men to fill their ranks. The French had to resort to using type A and Type B reservists to fill their ranks. Type A and Type B reservists were basically the washouts who weren't fit for front line military service. Type A were generally men who were too young an inexperienced to serve, where as Type B were older men in their mid thirties or over. When Alan Brooke (future British chief of staff) walked by these men he noted that the:

men were unshaven, horses ungroomed, clothes and saddlery that did not fit, vehicles dirty, and a complete lack of pride in themselves or their units. What shook me the most, however, the look in the men's faces, disgruntled and insubordinate looks. Although ordered to give the eyes left, hardly a man bothered to do so

The French Second Army which was crucial to stopping the German advance through the Ardennes contained numerous amounts of these Type B reservists. On the other hand Germany suffered from none of these problems, or at least dealt with them much more effectively than the French did. The German divisions were uniform in quality, and its fighting doctrine had been proven to be effective in the fight against Poland.

They were just behind the curve; the Germans were militarizing since basically 1933 and the French began very, very late. Even though they had "technically" better artillery and tanks, even more of them, ultimately they did not apply them in a modern fashion of concentration nor did they have an air force which could stand up to the Germans. So when the Germans applied a massive concentrated armored thrust through the Ardennes all that stood in their way were those Type B reservists, a very small amount of them at that, while the rest of the army was in Belgium meeting the rest of the German army and putting up a damned solid fight. No one thought, even the German generals themselves, an armored thrust through the Ardennes was feasible yet it worked masterfully and before the French knew it their entire Northern coast had German armor rampaging across it. They were totally encircled in Belgium/Northeast France and, with the British army, had their pocket condensed until the infamous Battle of Dunkirk.

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