Pope Francis has declared the use and possession of nuclear weapons to be “immoral” and demanded that possessors of those weapons give up their arsenals. His stance is a shift in the Vatican’s position on this critical issue: Three decades ago, his predecessor Pope John Paul II said that such weapons were acceptable as long as they were used for deterrence and steady progress was made toward nuclear disarmament. That prospect seems more distant than ever as nuclear powers discard the few remaining nuclear arms control agreements and are ready to embark on a new round of nuclear modernization.

Francis’ three-day stay in Japan was only the second visit to this country by a pontiff. John Paul II visited in 1981 and Francis retraced many of his steps. Francis began his Japan tour — his first stop was in Thailand — in Nagasaki, the cradle of Japanese Christianity, which spread from that city after St. Francis Xavier visited in 1549. He honored Christian missionaries and martyrs at the memorial of the 26 Nagasaki Martyrs who were crucified in 1597, the beginning of a campaign of anti-Christian persecution by Japanese leaders that lasted more than 250 years and drove believers underground.

While in Nagasaki, and again hours later in Hiroshima, Francis spoke out forcefully against nuclear weapons. His words were blunt: “The use of atomic energy for purposes of war is immoral,” adding that “the possession of weapons is also immoral.” He explained: “In a world where millions of children and families live in inhumane conditions, the money that is squandered and the fortunes made through the manufacture, upgrading, maintenance and sale of ever more destructive weapons are an affront crying out to heaven.”

With equal force, he rejected the rationalization for the continued possession of those weapons — the policy known as deterrence. Francis cut through the cant, noting that “Our world is marked by a perverse dichotomy that tries to defend and ensure stability and peace through a false sense of security sustained by a mentality of fear and mistrust.” He was right to conclude that “Peace and international stability are incompatible with attempts to build upon the fear of mutual destruction or the threat of total annihilation.”

Those words, delivered by individuals of moral authority such as the pope, are always powerful, but they assume special force in those two cities, the only two places on Earth where atomic weapons were used against human populations. Hiroshima was the first city to know the fury of the atom unleashed; when a U.S. plane dropped it on Aug. 6, 1945, 140,000 people died. Three days later, a second bomb was dropped on Nagasaki, resulting in another 74,000 deaths. For many others, survival was no less traumatic as they suffered from radiation poisoning.

Francis’ rejection of deterrence is not a new position. He first articulated it in 2017, and he has combined his moral authority with Vatican diplomacy: The Holy See was one of the first countries to sign and ratify the United Nations treaty prohibiting nuclear weapons. That shift reflects his personal rejection of the horrors of nuclear war as well as a pragmatic assessment of the state of nuclear diplomacy.

John Paul II’s willingness to accept nuclear weapons reflected the mindset of the Cold War and, more importantly, was conditioned on progress toward disarmament. Superpower nuclear arms control agreements followed and there was movement to reduce those arsenals. No longer. The Intermediate-range Nuclear Forces (INF) agreement has collapsed, the Iran nuclear deal is on life support, and the last surviving pact between the U.S. and Russia, New START (Strategic Arms Reduction Treaty), is likely to expire in 2021. The five officially recognized nuclear weapon states’ commitment to their nuclear arsenals has prompted other states to acquire their own capabilities. As a result, for the first time in over two decades, the number of nuclear weapons worldwide looks set to increase.

Japan shared the Vatican’s former position: preferring the abolition of nuclear weapons but accepting them as an ugly reality in a world in which they deter similarly armed adversaries. Francis’s new thinking puts the Japanese government at odds with the Vatican’s high-minded approach. That does not mean that Japan should shift as well — the pragmatic basis of its policy remains compelling.

But Tokyo should welcome the Pope’s full-throated endorsement of disarmament: It is critically important that the spectrum of views about nuclear weapons include a vocal and vibrant idealism. Japan should support diplomatic efforts to reduce nuclear weapons, continue to adhere to its nonnuclear principles and renew its pledge to remain nonnuclear.