Puerto Rico became a United States territory following the signing of the Jones-Shafroth Act by President Woodrow Wilson on March 2, 1917. This signing meant that Puerto Ricans could become U.S. citizens by act of Congress rather than through the Constitution.
The act gave statutory citizenship and that is why Congress had to grant citizenship to the people of Puerto Rico. This development enabled the people of Puerto Rico to join the U.S. workforce, including the military. The island contributed about 20,000 soldiers during World War I and became a vital base for U.S. forces during World War II.