Flamingo Bay, Puerto Rico, 1914
For one thing, Puerto Ricans born after April 25, 1898 were granted U.S. citizenship, although they had the option of rejecting it. Of 1.2 million residents on the island only 288 rejected citizenship.
The act also established a governmental system that closely mirrored those of the states, with a legislative, executive and judicial branch.
By bizarre coincidence, passage of the act made male Puerto Ricans liable for U.S. military service upon conscription just as the US was about to enter World War One.