Indeed, the French put the guns along their border to Germany, with a weaker section in the Ardennes, since it was deemed unpassable for mechanized units by the wisdom of the time.
The Belgians also fortified their border to match with the french, and the dutch did aswell.
The French and british belived that they main punch would be a repeat of the right hook through Belgium, executed during WWI, since the fortifications where not as extensive as the Maginot line.
The germans however, made a variation of their old plan, sending a wing smashing into Belgium and the Netherlands, using suprise attacks, ruses and airborne assault to take out for example the strong fortress of Eben Emael where they destroyed the heavy guns, allowing the armoured spearheads to get past unscratched. But the fortress itself held for a good time.
As the french and british field armies raced north to meet the german punch, a second german army attacked via the Ardennes, where the fortifications where weaker. Weaker fortification and the big suprise about such a daring move caught the french on the backfoot and after having battled their way out of the Ardennes, the german panzer divisons raced forward, while the bulk of the allied forces was in Belgium, thereby threatening them with catching them in a big sack and going for the almost undefended Paris.
Add to this that the germans gain a strong air superiority, which was just as important as their armoured divisions rapid advances. I belive that had the allied maintained parity in the air, the german advance would have been sloved, and it would have robbed them of much of the aerial recon they needed.
For example, on two occasions, german panzer divisions where caught strung out by first a french armoured counterattack, commanded by non other than Charles DeGaulle. It was initially succesful but german airpower saved the day.
The second is a more well known incident, more well known to the anglo audience cause it involved a british unit and that it was Rommels famed 7th "Ghost" division, that took the hit.
A british force of motorized infantry and armour, about two tank battalions, one with the machinegun armed Matilda 1, and another with the famous Matilda II, who at least had a 2-pounder cannon.
The massive ambush hit the germans in the flanks, as they where rushing forward, the machinegun armed Matilda 1 focusing on softskin vehicles, infantry and artillery, while the heavier Matilda II contested with the german tanks. The germans got absolutely freaked as they faced lots of tanks that their guns could not penetrate. Rommel ordered in desperation a unit of 88mm AA guns to open up, and finally managed to make a dent in the british onslaught. But in the end, it was once again the wailing Stukas of the Luftwaffe that shattered the british attack.
But this attack had one huge impact. The germans where not certain of how large the british formation had been, and overestimated the size to at least a division! Neither where they certain how many still battleworthy enemy armoured formations there might be around, ready to pounce on the strung out panzers. People say Hitler chickened out, he was later blamed by german generals for being to cautious there. However, the truth is that most of the german commanders agreed. They where not certain about all the enemy ahead, aka, they did not fully know their enemy. They knew that even if their units outclassed the french and british tactically, the french and british had tanks that outclassed their german ones. The best tank of the early war was the French Somua, a well built and well balanced tank, with a 47mm cannon and two machineguns, good mobility and good armour. The only drawback was the tower only big for two men, meaning the commander must also do some weapon duty.
More Somuas (it was a very new design) and the germans might not have been so happy. Also, the french airforce where also in a transition period, having just started introducing a new line of fighters. Among them where a design from Bloch, who was outclassing the german fighters in almost every way. In one air battle, the only french squadron that had been equipped with these new fighters, riped apart and opposing german Messerschmidt 109 squadron.
All in all, if you forget all kind of silly hindsights, the germans desicion to pull the brakes and resupply their panzers and allow infantry to catch up, was a sound military desicion. And that meant the british counterattack saved the BEF and a large number of french troops that was evaquated via Dunkirk.
Sure, the French could have fought better, but the germans just proved a bit more sneaky.
And if you are going to blame the french for misjudging the passability of the Ardennes to large armoured formation, well, then you should know that most allied commanders tought the same about it in December 1944. But we all love hindsight, don´t we.
