When did St Thomas Virgin Island became a territory?

St. Thomas, part of the U.S. Virgin Islands, became a territory of the United States in 1917 when the U.S. purchased the islands from Denmark. The acquisition was formalized through the Treaty of the Danish West Indies, signed on August 4, 1916, and the transfer took place on March 31, 1917. The islands were originally colonized by the Danish in the 17th century.