This is the ancestral land of the First People, the Kumeyaay.

For thousands of years, fresh water flowed down from mountains to the east, carving out a course and, with its sediments, creating the Tijuana River Estuary when it mixed with the waters of the sea. All that time, plants and animals adapted to the evolving environment, with Kumeyaay using the estuary and its surrounding land for food-gathering and habitation. With the arrival of Spanish colonists and soldiers in May of 1769, all those patterns began to change and the natural landscape was subject to a different idea of ownership and land management. This shift was cemented when in 1821, the Tijuana Estuary became part of Alta California. In 1848, the Treaty of Guadalupe Hidalgo ended the U.S.-Mexican War – and ceded present-day California to the United States, eventually solidifying borders that changed people’s ways of life.

You might have heard the war between the United States and Mexico referred to as the Mexican American War. We prefer to use the term U.S. -Mexican War because it acknowledges that Mexico is also a part of America, as well as the hard feelings that still exist to our neighbor to the south regarding that war.

In this episode of Divided Together, we’ll hear from Rachel St. John, an historian who wrote a book about the early U.S.-Mexico border period from 1848 to the 1930s. How did a simple line on a map transform into the regulated divide we have today? In this episode historian Rachel St. John shares how an array of officials, land pirates, and law enforcement created the foundations for the modern border control we have today.

Divided Together is a California State Parks podcast series for Border Field State Park, brought to you by Parks California and the generosity of an anonymous donor.

Adam Greenfield is the engineer and co-producer of this podcast. Anne Marie Tipton is the host and co-producer.