Here is the culprit!

Poster Number: PP 1006
Category: World War II
Poster Notes:

Poster is in Serbian language. Poster was issued by the German Army during their occupation of Yugoslavia.
The flags depicted are Great Britain, the United States and the Soviet Union, all allies during the war.

Media Size: 23x16.5
Poster Type: Offset
Publishing Date: c.1943
Technical Information on Poster: Pr B. XV/XI 5/235 Ba.
Catalog Notes: PP 1006 World War II b
Artist: Hanisch, Bruno —
Publisher: Publisher not indicated —