Ivan Jablonka's grandparents' lives ended long before his began: although Mates and Idesa Jablonka were his family, they were perfect strangers. When he set out to uncover their story, Jablonka had little to work with. Neither of them was the least bit famous, and they left little behind except their two orphaned children, a handful of letters, and a passport. Persecuted as communists in Poland, as refugees in France, and then as Jews under the Vichy regime, Mates and Idesa lived their short lives underground. They were overcome by the tragedies of the twentieth century: Stalinism, the...