Well, Puerto Rico status is traced back to 1898 during the Spanish American War. The US took Puerto Rico, the Philippines, and other Spanish territorial possessions by defeating Spain. Puerto Rico was under Military Government until the Commonwealth was established. Puerto Rico is considered as an State but not part of the US Union. Puerto Ricans are considered US Citizens.
Political Status of Puerto Rico has been a controversy for the past few years because 50% of the population wants to remain under the Commonwealth Status and the other 50% percent wants to joint the statehood of the US. There is a minority of the population that wants the independence. As some of the Caribbean Islands under British, French, Norwegian, and Dutch rule have been gaining independence, the American Government is entertaining the idea of granting Puerto Rico full Sovereignty in a near future.
A very condensed History of Puerto Rico.