Cities seeking to open sites where illegal drug users are monitored to prevent overdoses responded defiantly to a Justice Department threat to take “swift and aggressive action” against that approach to the nationwide opioid epidemic. Plans for those “supervised injection sites” — under consideration in San Francisco, Philadelphia, New York City, Seattle and elsewhere — collided with a stern Justice Department warning issued, threatening to create a standoff between federal and local authorities like the confrontation over “sanctuary cities.” As they have before, some liberal-leaning cities trying to cope with conditions on their streets find themselves at odds with more-restrictive Trump-era policy and enforcement.