Spain regulating presence of foreign submarines in her waters and harbours according to the Kriegsnachrichten of the Chef des Admiralstabes der Marine dated Berlin 11 August 1917

An item reported that according to a regulation of the Spanish cabinet dated 29 July 1917 submarines of the belligerents not allowed to operate in Spanish high sea waters neither to enter a Spanish harbor or they would be interned until the end of the war. Neutral submarines were still allowed to pass Spanish territory showing the proper flag.

Source

Bundesarchiv RM-40-622