If the Ardennes forest was heavily wooded as the Allies said, how did the Wehrmacht manage to penetrate it with an armored column?

 

The Ardennes was heavily forested. The question seems to imply that the Allies were just imagining the existence of trees; but no, they really were there.

There were narrow roads surrounded by wooded hills on both sides preventing the armoured columns from moving easily off-road. The region was a serious military obstacle.

As such, what happened in May 1940 was a bluff that failed.

It was obvious to everybody that if the Belgian army defended the Ardennes in force, they could stop any German offensive cold. A handful of anti-tank guns blocking the roads, with some infantry in the surrounding forest to prevent them being outflanked, could hold up an entire Panzer division.

The Belgians knew the Germans were aware of this, so they assumed the Germans would not even consider attacking that way — it would be too much of a risk.

So, if the Germans were not going to attack through the Ardennes, there was no point wasting troops by putting them there to defend it. It was safe to concentrate the small Belgian army somewhere further north where it would be more useful. The French army reached the same conclusion: while they would not leave the French border opposite the Ardennes entirely undefended, it was seen as a low-priority front that could be left to second-line troops.

Unfortunately, Manstein and Guderian called the Allied bluff. They guessed that the Ardennes would be left almost undefended, and so decided to send their main offensive through there. They were taking a huge risk: if the Belgians or French had left a stronger defence force there — or even if they’d reacted faster and rushed a strong reserve to the region while the Panzers were still moving through it — the result would have been a major German defeat. They were lucky, or they judged their opponents correctly, and the high-stakes gamble paid off.

(As a side note: when wargame designers create tabletop or video games based on this campaign, the Germans will almost always do badly unless the game includes special rules to handicap the defenders. With the benefit of hindsight, the Allied player will always block the Ardennes, unless the game artificially prevents them from doing so. Instead the fighting devolves into a slugging match in northern Belgium; and while the Germans still have the tactical advantage, the results are rarely anything like as one-sided as in history.)

In short: no army can be strong everywhere, especially if its opponent outnumbers it. Strategy demands concentrating forces instead of spreading them out. In 1940 the Allies gambled that the Germans would not risk going through the Ardennes; the Germans guessed correctly that they would make that decision, and called their bluff.