Turkey and the German Empire entered into a secret Ottoman-German Alliance on August 2, 1914 -- one day after Germany had declared war on Russia. This alliance ultimately led the Ottoman Empire to enter the First World War on the side of the Central Powers.
An important postal history ramification of the Alliance was the presence of German soldiers in Turkey who maintained their own fieldpost. The picture postcard shown here is an especially interesting example of that fieldpost because it was sent to Luxembourg.
The bilingual German fieldpost cancel reads 'FELDPOST MIL. MISS. KONSTANTINOPEL, 19-2-1916.' The card is endorsed "Feldpost" in manuscript at the top.
Upon receipt in Germany, the card was censored at Trier [purple cds] before being delivered to Differdange Usines, where the 5-centime domestic postcard rate was paid as postage due on March 1, 1916. The card view is shown at the top of this blog entry.