Did President Trump send the U.S. military to Los Angeles earlier this week to safeguard the city from protesters? Or is this the start of something bigger — an unprecedented push by the president to carry out mass deportations with the help of armed troops?

To some Angelenos, the notion that their city was being invaded by Mexico might sound like a fantastical twist on the history of U.S.-Mexico military relations.

"I urge the authorities to respect the right to peaceful assembly and to uphold human rights in law enforcement, including by refraining from any resort to military force when civilian authorities ar...