The anti-fascist counter-protesters who had turned out to oppose Saturday’s far-right march in central London had not anticipated being unable to leave. But more than six hours after they first gathered, they were trapped, surrounded on all sides by their opponents, who were kept at bay by lines of riot police. Seeking violence, many of the fascist thugs who attended the “Unite the Kingdom” march splintered off from the main 110,000-strong group after the initial march, catching the police off-guard by filing down sidestreets off Whitehall. When they came up behind the counter-protesters to take over Trafalgar Square, they cut...