In fact, in 1964 he led a retreat on the "Spiritual Roots of Protest." As Gordon Zahn said in his introduction to Thomas Merton in Peace, it must have been a powerful retreat. At least 5 of the attendees were later arrested for their "crimes" of resistance to the Vietnam War. But in his notes for the retreat, he makes clear that their time together was not intended to develop strategies of protest but to discern the call to resist within God's call.
“We are hoping to reflect together during these days on our common grounds for religious dissent and commitment in the face of the injustice and disorder of a world in which total war seems at times inevitable, in which few seek any but violent solutions to economic and social problems more critical and more vast than [the human person] has ever known before.
"What we are seeking is not the formulation of a program, but a deepening of roots. Roots in the ‘ground’ of all being, in God, through His word. Standing in the presence of His word knowing that we are judged by it. Bringing our inner motives into line with this judgement.” (Retreat, 1964, Spiritual Roots of Protest)
“Christ our Lord did not come to bring peace to the world as a kind of spiritual tranquilizer. He brought to His disciples a vocation and a task, to struggle in the world of violence to establish His peace not only in their own hearts but in society itself.” (Peace: A Religious Responsibility)
“A purely ‘spiritual’ witness is not enough. . . . We must certainly bring the world to repentance, but we must engage with the rest of [humankind] in a collaborative work of social renewal, reconciliation, in a serious effort to bring about a peaceful world situation, in which [people] can work together to solve the enormous social problems posed by the technological and economic revolution of our time.” (Christianity and Defense in the Nuclear Age)
“There must be a new force, the power of love, the power of understanding and human compassion, the strength of selflessness and cooperation, and the creative dynamism of the will to live and to build, and the will to forgive. The will for reconciliation.” (Preface to Vietnamese Translation of No Man is an Island)
“[I]t is necessary to go back to the sources and try to recover the true Christian meaning of the first and all-embracing commandment to love all men including our enemies.” (Saint Maximus the Confessor on Nonviolence)
“The religious basis of Christian nonviolence is then faith in Christ the Redeemer and obedience to his demand to love and manifest himself in us by a certain manner of acting in the world and in relation to other [people].” (Blessed are the Meek: The Christian Roots of Nonviolence)