Federal officers on horseback Monday morning, July 7, cruised a mostly empty MacArthur Park near downtown Los Angeles. Others in fatigues with helmets and long guns showed up, too. After an hour or so, they cleared out — defense officials had said some 90 California National Guard members would be there as well in addition to over a dozen military vehicles to help protect immigration officers during a raid at the park. City officials said they didn’t believe there were any arrests. The Department of Homeland Security wouldn’t say whether anyone had been arrested or what the operation was all...