QAnon reshaped Trump’s party and radicalized believers. The Capitol siege may just be the start.

The siege on the U.S. Capitol played out as a QAnon fantasy made real: The faithful rose up in their thousands, summoned to Washington by their leader, President Trump. They seized the people’s house as politicians cowered under desks. Hordes wearing T-shirts emblazoned with the “Q” symbol and toting Trump flags closed in to deliver justice, armed with zip-tie handcuffs and rope and guns.

The “#Storm” envisioned on far-right message boards had arrived. And two women who had died in the rampage — both QAnon devotees — had become what...