Eupen ist Frei
Eupen is Free

Quick history:
On 10th May 1940 the Eastern Cantons were occupied by Germany and a pro-German post office official overprinted a number of Belgian definitive stamps and postal with the words "Eupen ist frei" and a swastika. There is some evidence of postal usage, possibly "by favour". These stamps and postal stationery items are sometimes considered bogus. The German authorities subsequently issues special commemorative stamps.