French thinking was to force the Germans around to go through Belgium, and then stop them there (the speed of armored warfare wasn't anticipated). Of course it didnt work because the Germans found the Ardennes loophole.
Anyway, why didnt Belgium build their own fortifications? The political climate in the 30s was clearly tense and any rational defense military person could see war looming. Would it have provoked the Germans they thought? Or perhaps it was too costly for a small country, or perhaps they thought they were safe as they were neutral? Other reasons?
Thank you
They did build their own fortifications. While overall not as impressive as the Maginot Line, one has to consider that Belgium had arguably worse terrain to work with than the French. During the 1930s several forts were constructed, with the aim to stop or at the very least slow a German advance until British or French troops could reinforce defensible positions. The most notable of the forts was Eben-Emael, one of the largest modern fortifications at the time. In addition to the large forts, Belgium also constructed hundreds of pillpoxes alongside possible invasion routes, including the Ardennes. Unfortunately, the forts in general and Eben-Emael in particular were insufficiently prepared against attacks from the air, by either bombers or paratroopers/gliders. Instead of engaging the German Army in an artillery battle over long distances, the defenders had to fend off special forces that disabled the gun emplacements while on top of the fort. The fall of Eben-Emael meant that the Belgian defesive line along the Albert Canal was compromised, which made the defense of the country nigh impossible. I believe it is reasonable to conclude that the fortifications were outstanding by WW1 standards, but due to technological innovation and the relative surprise of the German assault, they failed anyway.
TL;DR: They did built fortifications, but they were not sufficient to stop the advance.
Sources: J E Kaufmann, H W Kaufmann: The Forts and Fortifications of Europe 1815- 1945: The Neutral States: The Netherlands, Belgium and Switzerland
Simon Dunstan: Fort Eben Emael: The key to Hitler’s victory in the West