On a stage festooned with Confederate flags, a singer was belting out “Dixieland Delight” by Alabama near an obelisk honoring the Americans who fled to this outpost in the aftermath of the Civil War. “We’re not racists,” said Cícero Carr, 54, an engineer whose great-great-grandfather hailed from Texas. Wearing a fedora featuring the rebel battle flag, he explained in Portuguese, “We’re just revering our ancestors who had the good sense to settle in Brazil.” At the annual celebration of Brazil’s self-described Confederados one scorching Sunday in April, Confederate flags adorned the hoop-skirted gowns of young belles and the trucker caps...