In the early 1900s, Teddy Roosevelt, New Mexico governors Miguel Antonio Otero and Octaviano Larrazolo, and Arizona legislator Carl Hayden—along with the voices of less well-known American women and men—promoted very different views on what being an American meant. Their writings and speeches contributed to definitions of American national identity during a tumultuous and dynamic era. At stake in these heated debates was the very meaning of what constituted an American, the political boundaries for the United States, and the legitimacy of cultural diversity in modern America.

In the early 1900s, Teddy Roosevelt, New Mexico governors Miguel Antonio Otero and Octaviano Larrazolo, and Arizona legislator Carl Hayden—along...