Over the two world wars, the Belgian brewing industry was probably more damaged them any other in the world. Including Germany. In WW I, most breweries were either blown to pieces by artillery or looted of their copper by the Germans. In WW II, they just ran out of war materials. It's a wonder that brewing survived at all.
Just as in the UK, bottles and casks were in short supply. Brewers couldn't afford to lose packages. With losses concentrated in the south of Holland, a finger was pointed at their closest neighbour.
"XII. LOSS OF CASKS AND BOTTLES
Mr. van Reede points out that the loss of bottles is great in the south of the country, while it is practically non-existent in the north.
Mr Stikker does not consider it impossible that bottles will disappear to Belgium.
Mr. Chambille says that it is not only a question of losing bottles, but also of barrels. It is therefore advisable to instruct the representatives of the breweries not to deliver casks that can be transported to Belgium and France."
Minutes of the management of the CBK on 14th November 1940, held at the Amsterdam City Archives, document number 31121-1, page 300.
Meanwhile, a lack of raw materials was really hitting home in Belgium:
"Mr Stikker agrees. In Belgium, too, beer gravity has been further reduced, with effect from 1 January to 7.5 and 2.5%. Nevertheless, Belgian breweries will probably run dry in about May. The result of this will probably be a redistribution of raw materials, at the instigation of Mr. Breedam, leader of the breweries on the agricultural front, who seems to feel more in favour of redistribution of raw materials than beer. At the moment a lot of German beer is sold in Belgium. With regard to possible further measures in the Netherlands, Speaker considers that an attempt should first be made to induce the authorities in Berlin to import malt into the Netherlands, for which purpose Speaker may possibly go to Berlin."
Minutes of the management of the CBK on 10th December 1940, held at the Amsterdam City Archives, document number 31121-1, page 281.
That's a reduction from around 3% ABV - just about intoxicating for the determined drinker - to 1% ABV. A beer you could safely give to a toddler.
In comparison, Holland was doing fairly well. Pilsner had fallen in strength, but was still 3.5% ABV. A decent session strength.