This is a passage from a glossy new history of Amstel. I picked it up at the Bokbierfestival.
The problems facing the Amstel and other Dutch brewers caused by the war seemed at first to be manageable. The year 1917 brought a tax increase and problems with raw materials which had far reaching effects for the Dutch beer industry. The huge tax increase on beer which Minister Treub introduced, meant that all breweries had to increase dramatically their retail prices in March 1917. In addition, stocks of malting barley and malt further declined, because of which Dutch breweries beers, in consultation with each other, decided to reduce the strength of their beer. From July 1917, the Amstel made only two types of weak 'war beer': light-coloured Pilsner and dark Lager. The higher prices combined with the deterioration in quality of the 'war beer' were not conducive to the domestic beer consumption. The export of bottled beer, which in previous years had caused an increase in sales had arranged, also collapsed.
The scarcity was a direct result of the unrestricted submarine warfare decalred by Germany on February 1, 1917. The risk of being torpedoed was great and ships were therefore not given permission by the Dutch government to fetch malt from the United States. Together with Heineken and Oranjeboom, Amstel then decided to charter a Danish ship. This Danish ship, the Maersk Olaf did successfully made the risky passage three times. The third load of to reach the Netherlands in this way was in June 1917 seized by the Dutch government, which forced the large breweries to give a portion to the small breweries. This ended immediately the risky trans-Atlantic malt charters. All Dutch brewers worked from mid-1917 through stocks in their malt. As the war dragged on, a collective decision to further dilute Dutch beer was inevitable. That decision was in April 1918. Following this decision, the quality of Amstel Pilsener and Amstel Lager declined even further. The recipes of these war beers differed significantly from those of pre-war times. In addition to malt, now rice, maize, tapioca and sugar were also used in the grist.
In the course of 1918 had many Dutch brewers had to stop beer production due to lack of raw materials."
"Amstel, het Verhaal van ons Bier 1870 - Heden" by Peter Zwaal, 2010, pages 59 and 66.
I like the bit about them having to use sugar and maize because of shortages. Later, of course, these adjuncts were totally standard ingredients in Dutch beer.