Why weren’t the Dutch and Belgian borders fortified with wire, trenches, mines, etc., as it became clear Germany would invade during WW2?

Question

Following standard military doctrine, a full mobilization of the opponent nation is the clearest indication of hostile intentions, short of an actual declaration of war.

Considering that massive, impossible to miss, military movements must have occurred, in the weeks and months prior to the invasion, among the German forces near and within the border areas, it’s hard to imagine what reasons could have led the Allied decision makers to not fortify the borders.

Was it a matter of cost? Political will? Or something else?

Answer 1

No, it wouldn't have worked.

... what reasons could have led the Allied decision makers ...

You're making a mistake here: before hostilities began, both The Netherlands and Belgium had (repeatedly) declared neutrality. Allied decision makers had no business there.

It was actually one of the reasons why the Maginot line eventually failed: In 1936 King Leopold declared his country unilaterally neutral, much to the surprise and dismay of France. That threw a big wrench in their Maginot plans. The planning relied on Belgium being allied with France. It would set up defences and build fortifications along the Diele river, to be occupied by Allied troops.

As Belgium now was a neutral country, nothing was built. The Allies only got access once hostilities began. It was even worse: after 1936 Allied officers weren't even allowed to visit those defensive positions to observe, let alone inspect them. Once the war broke out, they went to their assigned positions, finding nothing there.

As to your main question: it's technically possible to surround a country with barbed wire and fortifications, but very expensive. Not only that, but you have to man those fortifications. Both The Netherlands and Belgium simply lacked the manpower, the money and most of all the will to do that. In both countries, disarmament movements were very strong.

Strong fortifications wouldn't automatically make invasion impossible. The Germans made a glider assault on the fortress Eben-Emael, and took it. That fortress was one of the most advanced fortresses in the world, and considered impregnable. If such a fortress can be taken, less well defended positions can also be taken. (You can't be strong everywhere.)

As it was, the Dutch army planned to offer token resistance at the borders, and withdraw immediately to better defendable positions far away (The Netherlands is a small country; far is relative) from the borders.

Those positions (in Gelderland, Overijssel and Noord Brabant) had to be abandoned as well. Only the fortress of Kornwerderzand hold out. All the other positions fell after fierce fighting. The main line of defence was the Grebbe Line, which was overrun after 4 days of fighting.

Kornwerderzand was a modern fortification, build as a labour project during the crisis. The Grebbelinie was the main line of the defence, and not even finished before hostilities began. This was the main reason why Gen. Reijnders (in Dutch) didn't want to use that line. The minister of defence insisted, and fired him. Gen. Winkelman took over, and was obedient. In the end, Reijnders was right.

The Dutch attitude before the war began was a mixture of hope and despair: "We've always been neutral. It'll work this time as well." That was the crutch the 'gebroken geweertjes' (literally: broken rifles, the disarmament movement) used to prop themselves up. And despair: hopefully the Germans overlook us, we are more useful to them as a neutral country. The armed forces were woefully under-equipped, due to decades of neglect, disarmament and budget cuts.

When war became first a real danger and later inevitable, there wasn't enough time to mend things. For example, Koolhoven made a moderately good fighter, on request of the French air force. The French had to outsource fighter production, because production in France couldn't be raised. Few saw combat; most were destroyed when the Germans bombed the factory. Fokker made two decent fighters, the D XXI and the G1. The first was exported to Denmark and Finland, where they (in Finland) served for a very long time, and pretty well. Most of the G1 planes destroyed on the ground. They never got a chance to prove themselves.

Some factories paid for local air defence, to get at least some kind of protection. Gist-Brocades in Delft did that, and other companies followed. It didn't work, as it was far too little, far too late. Don't blame the Dutchies, though. This goes for just about every European nation at the time.

added:

General Reijnders had a valuable source of information: major Bert Sas, he was military attaché on the Dutch embassy in Berlin. Sas was in contact with col. Hans Oster, who gave him every time the date of the impending invasion of The Netherlands.

Unfortunately, that invasion was postponed, no less than 29 times. Not a fault of Oster or Sas, but it did made maj. Sas look like the boy who cried wolf. Gen. Reijnders didn't believe him to begin with. The 28 other times Sas informed him (later gen. Winkelman) didn't add to his credibility.

Reijnders and Winkelman simply couldn't believe Germany would invade. Until it did ...

Answer 2

Maginot, whose name has became synonymous with failure in France, and Petain - the symbol of national treason, were in fact quite competent and their plan of defense, on which they frequently clashed, was a sound plan. It required the extension of the Maginot line through Belgium which was obviously the weak point. Finally, it was the political leadership which dictated that the defenses were left with a hole in Belgium as France couldn't state at the time that they were abandoning Belgium in case of war (if they decided to fortify the French-Belgium border) or they were interfering with Belgium's desire for neutrality (if they decided to fortify the Belgium-German border). And Belgium never stood any chance of repelling a German attack without France.

Also, the German plan of attack was more or less known - the Manstein plan was in fact a slight variation of the Schlieffen plan - which dated from before WWI and it was the only reasonable vector of attack - and this plan was captured by the Allies 6 months before the invasion after the Mechelen incident.

Yet the Allies were ill-prepared. War had changed a lot in 20 years since the last one. They say that you always prepare for the next war as you fought the previous one. Read about the Eben-Emael fortress, it is very insightful. This was the very first airborne combat insertion and it was remarkably successful.

Also tanks pierce through anti-personnel mines, trenches and barbed wire without even stopping. It was exactly that kind of thinking that got the Allies in this situation - the new armored warfare was very different and defending was suddenly much more harder - because the enemy would simply concentrate armor on one weak point, break it no matter the static defenses and then infiltrate fast motorized divisions through this point and cut the supply lines. Then it is up to the original defenders to leave their trenches and attack. This is what happened in Northern France. Read about 'Schwerpunkt tactics'.

Answer 3

They were, as much as possible.

Belgium had forts with cannons on some of the rivers, like Eben-Emael, had units ready to defend the rivers, and the Netherlands had as well plans to defend rivers.

Both these countries had not enough money nor population to prepare fortifications to defend their Eastern border.