Early inhabitants of the Virgin Islands included the Ciboney, Arawaks, and Island Caribs. The first documented Europeans to visit the islands arrived with Christopher Columbus. The islands were occupied by several nations over the next century, including England, the Dutch Republic, France, and Denmark. In 1733, the Danish West India Company purchased Saint Croix from the French and brought together Saint Thomas, Saint Croix, and Saint John as the Danish West Indies. Danish trading posts were set up on the islands, trading in sugar, slaves and other goods. Sugar cane cultivation was a major economic activity for many years, with slaves used as one of the labor sources. However, following increasing humanitarian awareness, laws against slavery and a slave rebellion in 1848, the governor Peter von Scholten officially freed the last slaves the same year. The islands were purchased from the Danish by the United States in 1917 under the Treaty of the Danish West Indies.

Answer this question based on the passage: when did the us acquire the virgin islands? Let's have some stream of consciousness first.
The important part for answering this question is: The islands were purchased from the Danish by the United States in 1917 under the Treaty of the Danish West Indies. So, the answer is 1917.