People mourning the killing of Charlie Kirk carried candles and American flags in a solemn memorial last week at the Huntington Beach Pier, long a destination for conservative gatherings ranging from protests over pandemic-era lockdowns to rallies in support of President Trump. But on this night, things took a dark turn when dozens of men joined the crowd, chanting, “White men fight back.” Then on Saturday, a white nationalist organization, identified by experts as Patriot Front, showed up at another beachside memorial for Kirk. The men, wearing khakis, navy blue shirts and white gaiters concealing their faces, marched down Main...