For Javier Sinay, it all began with an email from his father in 2009. The message contained a link to an article Javier’s great-grandfather had written many years earlier. Mijl (Michel) Hakohen Sinay, with his parents and siblings, emigrated from Grodno to Argentina, where he eventually became a pioneering Yiddish journalist and writer. In the 1940s, Mijl had written an article titled “The First Fatal Victims in Moisés Ville,” an account of crimes perpetrated against vulnerable immigrants by unruly gauchos, as the South American horsemen were known. “Apart from everything it means to us emotionally and historically,” Javier’s father wrote...