Prosecutors say the plan was coordinated between men in at least six states, taking shape in mere weeks ahead of the June 14 event at the White House. By the time the police cruisers rolled up to the teenager’s home in Ohio one night in June, he and his alleged co-conspirators already had amassed guns, bullets and body armor. He told officers he had never met the others in person. He did not appear to know their real names. They had converged online a few months earlier and, according to court affidavits, soon were developing plans for bloodshed. They arranged...