As the U.S. celebrates its 250th anniversary, millions of Americans who live outside the 50 states are excluded from full participation in its democracy.

Roughly 3.6 million residents of U.S. territories – including Puerto Rico, Guam, American Samoa, the Northern Mariana Islands and the U.S. Virgin Islands – have no senators and only nonvoting representation in the House. These Americans, who can vote in presidential primaries but not the general election, are excluded because of where they live.

This year marks the 125th anniversary of the Insular Cases, a notorious series of Supreme Court decisions beginning in May 1901 that indelibly shaped the nation’s democracy. In these cases, the court decided that some territories were not, and would never be, an equal part of the U.S.

As political scientists who study the history of Congress, we’ve researched how lawmakers wrestled with the question of what rights to extend to the residents of overseas territories. Their answer shapes American democracy today.

After the Spanish-American War, fought over four months in 1898, the U.S. acquired vast new territories from Spain – including Guam, Puerto Rico and the Philippines – increasing its population by some 8 million people overnight with new residents thousands of miles from the mainland. Suddenly, the country was faced with a constitutional........

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