To Lose a Battle

France 1940 by Alistair Horne (orig 1969, updated 1990 with Ultra material).

Why did France collapse so quickly when Germany attacked?

He describes lots of reasons, including bad luck and bad weather, but the critical parts were

• Lack of preparation:
  • Communists: generically against capitalists at first, then once the USSR allied or close enough to it with Germany, actively sabotaging things
  • Politicians who hated each other more than outsiders were unable to cooperate on programs with no instant political reward
  • Political military leaders. One unexpcted side effect was multiple lines of authority. Another was lies about the true situation, and pretending you knew something when you did not.
  • Trust in their Maginot shield
  • Nowhere nearly enough R&D in aviation, nor capacity to build their own
• The lowland nations were terrified of seeming non-neutral, which meant they didn't fortify adequately against the Germans, which required battle plans that stationed lots of Allied troops ready to move into an unfortified northwest
• Lousy communications, with apparently little thought for resilience. During the battles, army groups couldn't communicate with their constituents nor with commanders at critical times
• Trench-warfare mentality. In a critical moment, when faced with Panzer attack, a French armored division had its tanks split up to cover a front rather than concentrated to counterattack. This seems to have been the default attitude for strategy as well as tactics--with some exceptions.
• Deception: The German attack through the Ardennes was covered by a "feint" in the northwest. The "feint" was a major attack in its own right, and successfully drew the reserves that way. The Germans broke through in the battle there as well. They feinted at the Maginot Line, and German propaganda threatened to come through Switzerland as well. The French left divisions guarding these that could have been better used elsewhere, though admittedly they weren't the best.
• Lousy morale.
• Few people had any clear idea about what tanks and dive bombers could do, especially together.
• Lack of practice with maneuver, making new air fields, and other such exercises

The German attack through the Ardennes was audacious and risky, and the drive west even more so. Near the end of his life Manstein, the inventor of the Sicelschnitt plan, said "The hopeless French reconaissance won us the Battle of France; just that."

Could be. The Germans had a better organized system, with coordinated air strikes and local air superiority. They might have won in the end anyway.

And Ultra didn't help. The Germans had changed codes, and at the start of the Ardennes strike were using land lines to keep anybody from eavedropping, so there was no way to warn of the start of the invasion, and by the time Ultra messages were getting decoded they were generally not timely, and communications were so poor that sometimes they never got through at all.

In short, the book explains what you probably learned in school, though in school we didn't learn about the mistresses. For a day by day coverage, with background, it's worth reading. Some of the points of failure have parallels today (e.g. political military leaders); some not so much.

It's useful to remember that however powerful you think your army is, somebody has a plan to bring it down--that might actually work. And that your leaders--civilian and military--are just men too, and therefore idiots. And that sometimes a genius one day is a fool the next.