This is likely a German M43 Field Cap. Introduced in 1943, this cap was issued across nearly all branches of the German military in an attempt to standardize headgear during World War II. It included side and back panels that could be folded down in cold weather to protect the ears and neck. While most regular army caps used two buttons, this single button variant was most prominently utilized by the Waffen-SS, the Luftwaffe, and the Hitler Youth.
