The French had built up Maginot Line, and the Germans had bypassed French defenses and attacked through Belgium at the start of the First World War. Why did the Belgium border not have more deterrents in place against Germany?
1) The political situation in Belgium.
Although we assisted the French in the occupation of the Rhineland our policy of strict neutrality was still in place. We even reconfirmed this after the Nazi's occupied the Rhineland in 1936.
This had several consequences for the deployment and the weaponry of our military. For example our army didn't have any real tanks because these were considered an offensive weapon. In addition our airforce was severly outdated. The lack of tanks was somewhat migitated by our effective artillery and anti-tank guns (C47 was perhaps one of the better anti-tank guns at the start of the war, and was sufficient to knock out any German armor in 1940).
2) The Plan
The Belgian defensive startegy was divided in 5 diffrent postions (Stelling).
Alarm Stelling: A combination of alarm and observation posts staffed with light infantry which would keep an eye on the German advance. This line went from Antwerp in the north, through Limburg and Liege till Luxembourg
Vooruitgeschoven Stelling: A bunkerline, on the canal between the Maas and the Schelde, made to delay the German advance while the main force of the Belgian Army would, eventually take op positions on the Weerstands Stelling. The line was staffed by the 16th infantry Regiment, our cavalry and the Ardeense Jagers. The plan was to destroy bridges, railways, tunnels, roads and other essential infrastructure.
Dekking Stelling: Similair to the Vooruitgeschoven Stelling A defensive postion behind the Maas and the Albert kanaal. The fortress of Liege and Namen were considered a part of the Dekkingstelling while the Albert Kanaal was reinforced by bunkers with machine guns. Ant-tank ditches were in place, all bridges were mined and several places would be flooded, again to delay the German advance. About half the Belgian Army would defend these positions before withdrawing to the Weerstand Stelling.
Weerstands Stelling: The main line where a German advance would be stopped. Roughly divided into 2 parts. From the fort Koningshooikt in the North till Louvain/Leuven would be the Belgian defensive postion while our allies would defend the positions from Louvain/Leuven till the French border. This line was heavily reinforced with bunkers, anti-tank ditches and fences, etc.
National Reduit Antwerp: From the Fortress Antwerp to the Bridgehead Ghent. Our last line of defense if all else failed. We would hold on to both Flanders while assisting our allies.
3) The Reality
The German assault, 10 May, managed to take the fortress of Eben-Emael and several key bridges in the opening hour of their offensive (the offensive was launched without and offical declaration of war.) From the start our army was pressed hard. The Dutch surrender after 5 days resulted in more German pressure on our northern flank. Combined with the succesful German gambit of driving through the Ardennes and crossing the Maas at Sedan with their armoured spearhead resulted in massive chaos for the Allied forces.
Due to the breakthrough in the South one position after another had to be abdoned by the Belgian Army. Everytime we had to destroy more and more of our own supplies to keep them from falling into German hands. By the 14th of may, only 4 days after the invasion, it was clear that our principal line of Defense (the Weerstandstelling) would be compromised and by the 16th we started evacuating this line for other positions further west. Our airforce was nearly completly destroyed by then so the retreat was mostly done during the night. Our lack of armor also made it impossible to launch succesful counterattacks. The Wehrmacht had a complete monopoly on mobility.
By the 18th of May our army had succesfuly retreated to the line Terneuzen, Ghent, Oudenaarde and although their were no defensive positions in place at the start of the war we managed to repell several german attacks. Belgian Morale was low though. The fierce and rapid german advance with their tanks and Luftwaffe was unprecedented, good defensive postion after defensive position had to be abandonned due to the danger in the south and we ended up in makeshift defensive positions, in addition we had to destroy massive ammounts of ammunition before each retreat to keep them from falling into German hands which resulted in a lack of supplies. But even with al these setbacks the Belgian front was stabilising but the german advance in the south remained worrisome.
When the Wehrmacht conquered Abbeville on the 21st of May our army again had to retreat. Again we left our defensive position for even worse one, again we had to destroy several stocks of weaponry and ammunition and this time the Wehrmacht was in close pursuit. By the 26th the Belgian army was in complete dissarray, had lost its connection to the Allied forces, was overwhelmed on all fronts and nearly encircled. The Belgian King, commander in chief, would capitulate the next day.
The French considered this a great betrayal:
On May 28th, 1940, Premier Paul Reynaud of France made the following statement over the radio: "I must announce to the French people a grave event. This event occurred last night. France can no longer count on the Belgian Army King Leopold III without a word to the French and British soldiers laid down his arms. It is a fact without precedent in history."
Which wasn"t exactly true but it's unclear how accurate the information was Reynaud received from his own army.
While Churchill was more nuanced in his address to the house of commons:
"I have no intention of suggesting to the House that we should attempt at this moment to pass judgment upon the action of the King of the Belgians in his capacity as Commander-in-Chief of the Belgian Army. That army has fought very bravely and has both suffered and inflicted heavy losses."
The Belgian army had suffered about 7000 deaths, more then 55.000 wounded and 50.000 prisoners in 16 days which is roughly one fourth of the Belgian army.
Sources:
and my own expertise as a guide for the War Heritage Institute (of Belgium)
TL:DR: Although there were defensive fortifications/plans our options were limited due to budgetary and political constraints. The plans/fortifications that were in place had to be discarded due to the rapid advance of german armour through southern Belgian/Northern France. Several tactical retreats later led to an underequipped, demoralised army on the brink of destruction resulting in a Belgian capitulation.