Bonhoeffer studied theology in Germany and the U.S. and was involved in the ecumenical movement. Shortly after Hitler came to power in 1933, he helped to organize the "confessing church," a Christian movement to resist Nazism. Illegally, Bonhoeffer trained pastors who refused to comply with Nazism and anti-Semitism. His most important book, The Cost of Discipleship, called Christians to follow after Christ no matter what the consequences. Bonhoeffer traveled to New York in 1939, and could have safely remained there--but he chose to return to Germany. The Nazis restricted his activities and watched him closely. In 1943 he was arrested and moved to various prisons and concentration camps. On April 9, 1945, less than a month before Germany surrendered to the Allies, Bonhoeffer and several other resisters were hanged by the Nazis.