Where was the original border between Mexico and the United States after Mexico gained independence from Spain in 1821?

To pinpoint the exact location, a team backed by a museum in San Diego and guided by GPS information spent weeks in 2014 traveling the original international border. Their findings fill in the blanks created by textbooks that omit or gloss over the boundary between the USA and Spain established by the Adams-Onís Treaty.

The project started with the question, “What would Mexico and the United States look like if that boundary had been fully realized?”

Along their route, team members erected 47 markers to show the original border. Marker #01 is on the Pacific Coast near the state line between California and Oregon. The team erected markers running along northern state lines of Nevada and Utah, across a lower portion of Wyoming, southward through Colorado, Kansas, and Oklahoma, and to Texas and Louisiana. Marker #47 is on the Gulf Coast near the state line between Texas and Louisiana.

During their journey, they encountered dozens of US residents curious about the project and eager to learn more about border history. All were friendly—one woman invited them to erect a marker in her yard, and one man gave them a place to stay one night. “Curious amazement best describes the reactions,” says team member David Taylor.

A narrative included in the exhibit states that only a few people the project team met seemed to grasp that Mexico once encompassed all of present day Arizona, California, Nevada, New Mexico, Utah, Texas, more than half of Colorado, and smaller portions of Wyoming, Kansas, and Oklahoma. One marker is near grain elevators on the edge of Dodge City, which might have become a border town today if not for the Mexican-American War. Who knew?

Many textbooks that marginalize the significance of the 1821 boundary also hide key aspects of the war that took about half of Mexico by military force—one of the largest land conquests in modern military history. This misleads many into thinking the land acquisition was something like the Louisiana Purchase.