During WWI, Spain was a neutral country. As a result, mail from Allied and Entente governments initially flowed rather freely through the country. However, following the German re-introduction of unrestricted submarine warfare in February 1917, and the resulting sinking of numerous Spanish vessels, Spain began limiting German mail traffic through the country.
Due to these limits, German agencies in Spain began sending mail to Germany via U-boat.
This cover was sent from the German consulate in Seville, Spain, to a bank in Braunschweig, Germany. At that time, banks often acted as intermediaries for the mail. A note accompanying this cover indicated that the eventual recipient was assigned to a Coast Guard unit on the island of Norderney in the North Sea, and the sender was the recipient’s father who worked at the Consulate in Seville.