And, then, 1914.
Germany has a naval base in the Caribbean during World War I. A direct threat to neighboring Puerto Rico, should the U.S. have entered the war as it did. It primarily would have housed commerce raiders but could have done more.
Imagine that the Kaiser, or somebody, would have said, "Yes, submarines aren't sexy, and Captain Mahan might not have incorporated them in his book on sea power, but they're cheap, they're a new, disruptive weapon, and a submarine base in the German Virgin Islands will allow them to roam all across the North Atlantic.