My first campaign I diploannexed Luxembourgh and blitzed throug Metz and took out France w/o any trouble.
Next game Metz was occupied with +20 divisions so I went through Belguim doing exactly what walrus suggested:
"When starting your attack on the Western front try a simeltaneous attack on Arlon & Brussels. Remember that your infantry is going to be slaughtered if it has to attack across a river (into Brussels) and that it will also suffer big losses against land forts.
The province you really want first (in my opinion) is Arlon, because if you let a big stack form in there you will have problems shifting it - that will happen if you force a belgian retreat from Brussels without taking arlon first.
So hit Arlon in good warm weather with 24+ mech and armoured units and throw what armour you have left at Brussels with some cannon fodder infantry support. You can always retreat from Brussels once the battle in Arlon is going your way - the objective of the Brussels attack is just to stop the Belgians sending troops into Arlon during your battle there. Once Arlon is yours you can launch a 2-pronged attack on Brussels at your leisure.
Then roll onto Sedan and try and encircle the Lille stack. That should give you a foothold and it's up to you how to progress from there."
Well I tried the direct follow up attack after taking Arlons with some 80 divisions pursuing the 10 or so defending Belgians that were more or less dead from the Arlon stomping and the 10 Brussel defenders that were messed up from my 30 division diversion attack into Brussel. Good weather, leaders and air support, multi-direction attack. I was crushed. 80 against 20 half dead divisions.
I rested until my troops were fully recovered, pinned down neighbouring provinces, sent in all my air and charged 110 divisions against 30, now rested, Belgian divisions in Brussel from Arlons and Köln. I lost 1000 strength points to each point the Belgians lost. Good weather, leaders and air support multi-direction attack.
These attacks would have been easily won in 1.04. Here it was impossible to gain ground. I agree that you have to lurew the Frencg far into Germany or repidly attack where the AI leaves a gap, which it seldom does on a 5 province front with 180 french, 120 UK and 30 Belguim divisions at its disposal.
Italy had landed large numbers of troops in the south so the allies were quite busy there...
I guess what Im saying that the west front with its narrow span, fortified provinces and rivers can be really hard to pass. Maybe paratroopers or naval invasions is the solution.