SAN DIEGO—Between the primary and secondary border walls where the U.S.–Mexico border meets the Pacific Ocean, a squad of Marines in full combat uniform and armed with M27s in two civilian trucks approaches the beach at Friendship Park.

Straddling San Diego and Tijuana, Mexico, the park was, until 1993, separated by a small fence so people living on both sides of the border could meet. What has changed in the area since then is a window into U.S. border policy through the decades.