No sooner had the U.S. Revolution ended than U.S. expansionists began looking south and southwest toward lands controlled by Spain.

The personification of this complicated project was the American poet-politician Joel Barlow. As a poet, he worked on creating public sentiment to annex Spanish lands, and, wearing his political hat, he pursued that agenda. Barlow is largely a forgotten figure today, but the myths he helped create remain with us.

Barlow was the likely author of the 1792 manuscript “Plan …