An aborted assassination plot against Hitler resulted in Bonhoeffer's imprisonment. During the last two years of his life he became something of a mystic, writing Letters and Papers from Prison. This book influenced American Christianity in the postwar years. He was upset that the German state churches had given in to Hitler. He was a member of the state church which even in pre-World War II Germany was ecumenically minded.
Sinfulness leads to wars.
Whatever the outcome of war, it must be the will of God. Notice how intimately involved the will of God always is in the rise and fall of nations and the individuals who live in those nations. Therefore, even where God permits the sin of mankind to lead to the bloodshed and cruelty of warfare, there God still accomplishes his will. However, even though God permits a war and uses it to accomplish his will in this world, that does not excuse the sin that may have been present in the motives and purpose of those involved in a war. Just because one nation won a particular war, that certainly does not mean that their cause was just or that God was on their side.