According to Bonhoeffer “Everything changed” when  he discovered the Sermon on the Mount:

I was lacking in humility…terribly ambitious…alone…then something happened…for the first time I discovered the Bible…Then the Bible freed me from that, in particular the Sermon on the Mount. Since then everything has changed. I have felt this plainly, and so have other people around me. –Ferdinand Schlingensiepen,  Dietrich Bonhoeffer 1906-1945: Martyr, Thinker, Man of Resistance pg.95

Bonhoeffer scholar Ferdinand Schlingensiepen says the influence of Jean Lasserre was influential in Bonhoeffer’s transition from theologian to Christian:

Bonhoeffer’s ‘transition from theologian to Christian’ …was induced by Bonhoeffer’s experience in the USA, particularly his conversations with Jean Lasserre (A Christian pacifist who believed Christians should live the Sermon on the Mount today) –Ferdinand Schlingensiepen,  Dietrich Bonhoeffer 1906-1945: Martyr, Thinker, Man of Resistance pg.94

In 1935 Bonhoeffer still believed the Sermon on the Mount was the hope of the church:

The restoration of the church must surely depend on a new kind of monasticism, which has nothing in common with the old but a life of uncompromising discipleship, follow Christ according to the Sermon on the Mount. I believe the time as come to gather people together and do this” (Bonhoeffer in letter to his brother in 1935) –Ferdinand Schlingensiepen,  Dietrich Bonhoeffer 1906-1945: Martyr, Thinker, Man of Resistance pg.175

How should the Church celebrate Memorial Day? Some thoughts from Bonhoeffer’s 1932 Memorial Day sermon:

The demons rise up. It is a rebellion against Christ. And one great power of this uprising is called war! …

“What does faithfulness of the church-community of Christ mean here other than calling out into this furious raging [of war] again and again – unto exhaustion, unto humiliation, unto martyrdom – the words of Christ that there should be peace, that there should be love, that there should be blessing, and that he is our peace, and that God is a God of peace?

“And the more they rage, the more we should call out. And the more we call out, the more wildly they will rage. For wherever the word of Christ is truly spoken, the world senses that it is either ruinous madness or ruinous truth, which endangers its very life. Where peace is really spoken, war must rage twice as hard, for it senses that it is about to be driven out. Christ intends to be its death.” —The Collected Sermons of Dietrich Bonhoeffer, Dietrich Bonhoeffer (editor: Isabel Best) via TGC

What did Bonhoeffer think was necessary to destroy Nazism? Assassination? Living the Sermon on the Mount?

I would only achieve true inner clarity & honesty by really starting to take the Sermon on the Mount seriously. Here alone lies the force that can blow all this hocu pocus [of Nazism] sky high – like fireworks, leaving only a few burnt-out shells behind…  (Dietrich Bonhoeffer in a letter to his brother Karl) –Ferdinand Schlingensiepen,  Dietrich Bonhoeffer 1906-1945: Martyr, Thinker, Man of Resistance

Bonhoeffer has well documented his confidence in living the Sermon on the Mount in “The Cost of Discipleship” (in which Bonhoeffer is more radical about cheap grace & the living the Sermon on the Mount than many Mennonites I have met!) . I have scoured his later “Ethics” to find any trace of a reversing of his earlier strongly held beliefs and I can find none.