Was Jan. 6 the end of an era – or start of a dangerous new one?

To Americans weary of political tumult, a presidential transition came with hopes for renewed stability. Yet experts say radical extremism has been rising and has deep roots. Solutions will require persistence.

Hours after President Joe Biden’s heavily defended inauguration and call for national unity and civility, about 100 anti-fascist protesters, many hooded and clad in black, vandalized federal and corporate buildings in downtown Seattle, burning a large U.S. flag, smashing windows, and spraying graffiti with messages such as “kill ICE” and “abolish everything.” They also hurled abuse at the new...