The American Civil Liberties Union (ACLU) on Thursday filed a federal lawsuit on behalf of Sam O’Hara, alleging that his constitutional rights were violated when Metropolitan Police Department officers handcuffed and arrested him while he was “peacefully protesting” against the troop deployment in D.C.

“The law might have tolerated government conduct of this sort a long time ago in a galaxy far, far away. But in the here and now, the First Amendment bars government officials from shutting down peac...