The Pope's Last Crusade: How an American Jesuit Helped Pope Pius XI's Campaign to Stop Hitler

A conspiracy within the Vatican—to stop an outspoken Pope

In 1938, Pope Pius XI was the world's most prominent critic of the Nazi Party. To make his voice heard, Pius called upon an American Jesuit, John LaFarge, to write a papal encyclical—the Vatican's strongest decree—publicly condemning Hitler's murderous campaign.

But conservative members in the Vatican like Cardinal ­Eugenio Pacelli toiled in secret to suppress the document from appearing. Pacelli, who would become Pope Pius XII, collud...