L’Eupen-Malmédy is another contended region of Europe – under Prussian governance until the end of World War I, it was annexed to Belgium during the Paris Peace conference, a choice ratified by a controversial referendum in 1920. It was again annexed to Germany during World War II, and returned to Belgium in 1945. Today it is the largest German-speaking area of Belgium.
These documents come from the Museum of Contemporary Worlds in Nanterre, but you can find everything on Eupen-Malmédy at the State Archives of Liège in Belgium
Conférence de la paix – Annexion de la région d’Eupen-Malmédy à la Belgique : coupures de presse. Papillon. Tracts belges. La manifestation Baltia à Malmédy (dans L’Etoile belge). Tract sur Malmédy.
Press cuts on the annexation, 1920ca
Archives Portal Europe Blog 
Germany took Eupen-Malmedy back during their occupation of Belgium in 1940-44. The new border went further than the pre-1914 border. Strangely enough, the map with the new border was not ment to be published. https://twitter.com/VerbanckSteven/status/1427618453796556812
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