One week before scores of Proud Boys helped lead a pro-Trump mob in a violent assault on the Capitol last year, Enrique Tarrio, the chairman of the group, and some of his top lieutenants held a foul-mouthed video conference with a handpicked crew of members. The meeting, on Dec. 30, 2020, marked the founding of a special new chapter of the Proud Boys called the Ministry of Self-Defense. The team of several dozen trusted members was intended, Mr. Tarrio told his men, to bring a level of order and professionalism to the group’s upcoming march in Washington on Jan. 6,...