On July 4, the nation’s 250th anniversary, a mysterious group of young men in khaki pants, navy blue shirts, logoed baseball caps and white face masks appeared in the capital, marching out of a downtown Metro subway station in Washington and into the news.By now, they look oddly familiar, part of the menagerie of uniformed street brawlers and torch carriers that emerged in the tumultuous first term of President Donald Trump. And there are unsavory historical echoes as well. Though smaller in number, the neo-Nazis in the capital brought to mind the 1925 parade down Pennsylvania Avenue by thousands of...