I think it was when he decided to return to Germany from the United States in 1939…

Perhaps the inner conflict between bourgeois security and radical Christianity reached its most acute point in the summer of 1939. Faced with the imminent prospect of draft into Hitler’s army, Bonhoeffer accepted the invitation of Reinhold Neibuhr and Paul Lehmann to embark on a lecture tour in the United States. This was an opportunity to live in safety, to write and publish freely, and perhaps find a respite for his conscience as war clouds thickened over Europe. Earlier Niebuhr had successfully rescued Paul Tillich from his political dilemma in Germany. And now another of Germany’s brightest theological minds was arriving in the port of New York.

The opportunity afforded by bourgeois society were surfacing once again, but this time Bonhoeffer could find a salve for his troubled conscience. Arriving before 13 June, he sensed he had made a bad decision.

He managed to stay only a little less than a month. 

 (Craig J. SlaneBonhoeffer as Martyr: Social Responsibility and Modern Christian Commitment19)