Bangladesh, Sept. 17 -- Once a marginal figure on Britains far-right fringe, activist Tommy Robinson drew a crowd estimated at 150,000 to a weekend march in London, leaving the country grappling with the implications of the mainstreaming of extremist politics. The event, which took place on September 13, sent shockwaves through the media and political establishment, highlighting deepening societal divisions and the potency of a populist message resonating across class and regional lines.

Robinsons ability to mobilize such a massive turnout is particularly striking given his controversial reputation, criminal record, and history of aggressive behavior. Experts point to a “perfect storm” of factors that enabled this surge in su...