District Attorney Jenkins said she did not envision police officers handcuffing federal agents in full view on city streets. Instead, she said, local law enforcement could review camera footage of beatings, if they occur, and try to identify the agent involved. Then, she said, she would ask a judge to sign a warrant for the agent’s arrest and seek to prosecute the agent in court.

“For me, this is about San Francisco and what I need to do for San Francisco,” she said.

That could be much easier said than done. Many federal agents are operating in masks and without badges or other identification; a California law barring agents from wearing masks does not take effect until January and even then is quite likely to face legal challenges. And the Trump administration could seek immunity for its officers or ignore the warrants altogether, Ms. Jenkins acknowledged.

If you’re not going to arrest them on the spot, it’s already a failed plan. They wear masks for exactly this purpose.