It would have been effective in ww2 too. However they trusted that the Belgian will defend their short borders, and not surrender like in ww1.
Belgium however did not even really notice the Germans running through their country. Then Belgium surrendered since the Germans were already going through their country and they had no chance of winning any fight (also France did not give them any real support against Germany).
Once in France the Germans mainly ignored the Maginot line or attacked it from the back where it was almost not inforced.
PS: this is what I remember from school history. Probably got a few things wrong.