Little-known fact: with the Nazi armies rampaging through the Low Countries, the French government decided to actually give command of their military to a competent general (no, not De Gaulle), namely Maxime Weygand, architect of the Warsaw Miracle and retired at the time. He actually came up with a plan to salvage the situation: catch the overextended German forces with a pincer move, hammering them against the British anvil. British high command agreed, as it did look like it would work.
In fact, with the benefit of hindsight, military historians think that, well… yeah, it would have totally worked. But we’ll never know for sure because the British general panicked and basically ordered the BEF to flee for their lives, leaving piles of equipment behind and blowing up bridges on the way to Dunkerque. (Hence the frankly unfair joke, “British rifle - never used, dropped once”) The French forces, in an untenable position with their flank suddenly gone, tried their best to fight an orderly (if very costly in lives) retreat, to slow the German as much as possible to cover the sea evacuation.
Then again, pinning the responsibility of the defeat on this action may be harsh: after all, if the French high command had listened to Weygand (again) and actually put some defences in the Ardennes, or even if the officer responsible for this sector hadn’t repeatedly ignored the very clear reports of tank columns in a giant traffic jam there and then explicitly told the Air Force that there was no need for tactical bombardment or ground support there, or if the French government of the Interwar period hadn’t decided to underfund and undercut the army for political reasons and appoint generals for political allegiance instead of competence, or if the German plans hadn’t randomly fallen in Belgian hands, driving them to change them and attempt an attack through the Ardennes, or if Belgium hadn’t left the Allies after their lack of reaction to the remilitarisation of the Rhénanie, stopping Allied armies from entering Belgian territory and position on the much more defensible frontier instead of rushing on after war is declared, or if Belgium had done it sooner and left enough time for France to complete fortifications on the Belgian frontier, or if interwar France had accepted to lend the money to Belgium that they needed to complete their own end of the Maginot line on their frontier, or if British and French hadn’t dropped doctrinal development of tank and combined warfare right after WWI, or if one of them had realized that appeasing Hitler wasn’t going to work (then again, it would have meant reading Mein Kampf, which is universally recognized as an epitome of awful writing style, so they can be forgiven) and mobilize as well as letting Poland mobilizing…
Well, if any single one of those things hadn’t happened, and I’m probably forgetting quite a few, it would have ended up in an early Allied victory.
Combine that with the spooky luck Hitler had at surviving assassination attempts, its forays in occultism, the pure-industrial-grade-evil regime and the luck deserting him once things have taken an appropriately apocalyptic scale, and seriously someone has got to write a book explaining that with Hitler having made a Deal with the Devil, right? I mean the thing practically writes itself at this point.