Lt. Col. Phillip Mason stood on a narrow peninsula in early April just west of downtown San Diego, overlooking the coastal border between the U.S. and Mexico as a seasonal fog obscured the view.

The end of British colonial rule birthed two sovereign nations—but hastily drawn borders caused simmering tensions to boil over. 75 years later, memories of Partition still haunt survivors.

Customs and Border Protection has quietly ordered its agents to steer clear of events hosted by organizations that support women or minority groups in law enforcement, a senior official said, The New ...