Belgium comprises, more or less, OTL Belgium.
Belgium was invaded by the Third Reich, surrendering in May 28, 1940. It was soon partitioned with the north (Flanders) included in the Reich Land of Niederlande, and the south (Wallonia) annexed by (Vichy) France.
In 1955, the new French government recognized the independence of Wallonia, and the monarchy was restored.
In 1986, the independent government of the Netherlands recognized the independence of Flanders and Luxembourg.
The 1st January, 1988, both Belgiums, Wallonia and Flanders were officially reunited into the Kingdom of Belgium.