In August 1914, after the King of the Belgians Albert 1st and the Belgian government reject the German ultimatum, Germany invades Belgium and, by October, occupies nearly the entire country. Though each belligerent still thinks that the war will not be long, it's out of the question for the German high command not to methodically organise the conquered lands. Indeed, it would be very dangerous not to totally control a territory that now finds itself behind German lines, and that will serve as a conduit for reinforcements in terms of the manpower, food and materials needed for combat.
While the first few weeks see the provinces being administered by the troops that had conquered them, it quickly becomes obvious that a more general organisation is required. The first governor general is appointed at the end of August 1914. He is quickly followed, in November, by the man who would control the destiny of the Belgians for more than three years, General von Bissing.
To avoid excessive upheavals for the civilian population, the overall organisation of the Kingdom's administration is initially retained. Each province has a military governor, and while the ministries retain their prerogatives and assignments, they operate under the very strict control of the German army. With this in mind, Germans are dispatched to Brussels in order to dominate the administrative system and to set the tone for how the occupied Belgians are to act.
But the new German administration quickly decides to rely on the old notion of "divide and conquer". Von Bissing, whose gift to posterity will be a testament for Belgium in which he describes his long-term vision, embarks upon a grand destructive policy. Belgium, that had developed an unprecedented sense of patriotism and that had come together as one behind its army, its King and its government, had to be ripped apart at the seams in order to be better locked down. In his plan for Belgium, von Bissing viewed Wallonia, as well as northern France, as a reservoir for manpower and raw materials, while Flanders would be a more Germanic protectorate and therefore more easily assimilated into Germany.
The government then launches its Flamenpolitik with, starting in 1916, the "dutchification" of the University of Gand and a degree of political support for the Flemish nationalists who will actively participate in the 1917 set-up of the first regional institutions. The entire administration will be divided into 2 very distinct linguistic regions, one Walloon and French-speaking based in Namur, and one Flemish based in Brussels. For its part, Brabant will also be split in half and divided between the two regions.
While the German governor general retains operational control for all of occupied Belgium, one cannot truly speak of any federalisation as we know it today: all ministries are divided, and the laws are henceforth different between the 2 linguistic regions. In November 1918, when Belgian authority is restored to the entire territory, these measures will be declared null and void and, for another 10 years or so, the individual ministries will once be reunited.