-
7 Fort Willoughby in the U.S. Virgin Islands

For many centuries Europeans fought wars over the accession of the Virgin Islands, these Caribbean islands were claimed and fought-over and contested by a succession of European powers, until the U.S. Virgin Islands joined the USA, in 1917. Very interestingly Fort Willoughby, on tiny Hassel Island, may be the only fort in the USA that has served both the Danish and British militaries but Americans forces never used this Fort.