For the first decades of Santa Fe Trail use by Americans, this was the border and Mexico was on the left.

In 1821, William Becknell and a handful of MO-based traders leading pack animals were warmly welcomed by Mexicans who had just won independence from Spain. The next year, Becknell and others returned with loaded wagons.

Borders as we know them today with fences and GPS coordinates did not exist then. Mexico struggled from the start and could not hold the northern frontier areas it claimed against a coming surge of Americans.