It has been nearly 58 years since the last star was added to the U.S. flag when Hawaii joined the union in August 1959. In a referendum 10 days ago, Puerto Rico residents voted to become the next. The matter isn’t up to residents alone, of course — Congress and the president would have to sign off on the admission of a new state, and there are many political hurdles that stand in between America’s closest possession and statehood. But those obstacles existed for Alaska, too, and we had allies who helped us gain entry to the U.S. As Puerto...