Pope Francis reemphasized opposition to nuclear weapons, the global arms trade, and capital punishment on the plane ride back to Rome following his apostolic visit to Japan and Thailand.

During his visit, Francis visited both Hiroshima and Nagasaki--locations where the United States dropped atomic weapons at the conclusion of World War II. Francis said he found that Hiroshima was "a true human catechesis on cruelty: cruelty."

"There I have reiterated that the use of nuclear weapons is immoral," said Pope Francis. "This must go in the Catechism of the Catholic Church." The pontiff clarified that he is opposed not only to the use of these weapons, but also a country possessing them. A country possessing nuclear weapons could one day be ruled by someone who could use them to destroy humanity, he said.

Pope Francis also was cautious on the topic of nuclear power, saying that he does not believe it should be used until it can be made safer, due to the potential for serious human and environmental costs of a disaster.

"The security," said Pope Francis, "has not been worked out. I, and this is my personal opinion, I would not use nuclear energy until there is a total security of use."

"We must do research on safety, both regarding avoiding a disaster and on the environment(al consequences," he said. "Caring for the environment is something that [we must do] today or never."

Many energy scientists say that nuclear energy is among the safest and cleanest form of power, though when disasters do occur, they are dramatic, and the cost of cleanup is high. Italy is one of two countries that have completely phased out the use of nuclear power, and the public does not have a strong perception of nuclear energy.

The pope also spoke about Japan's use of the death penalty, and was asked why he did not reference the death penalty during his speeches in Japan. Japan, along with the United States, is one of the last remaining first-world countries with capital punishment.

Pope Francis said that he had not been aware that a prisoner that had been sentenced to death was present at his Mass at Tokyo Dome, and that he had "spoken in general of many problems of condemnations, of eternal processes that never end, both with death and without death."

"Fifteen days ago I made an intervention at the convention of international law, and I talked seriously about the subject of the prisons of precaution and then the death penalty, which was clearly said that it is not moral that cannot be done," he added.

Prisons, said the pope, must treat the imprisoned with dignity and respect, and prepare them to reintegrate into society once they have served their sentences. And while the death penalty is not yet abolished worldwide, Francis believes that steps can be taken to further this process.

"We must fight against the death penalty slowly," he said, acknowledging that an outright abolition of the practice in some countries is simply not feasible due to "political problems." Instead, a country could simply suspend the practice and revert those on death row to sentences of life in prison.

Pope Francis was then asked to comment on the issues of self-defense, just war, and if he was planning on releasing an encyclical on peace. The pope responded that an encyclical on that topic is not quite ready just yet, and could be left to his eventual successor.

Violence, said the pope "is a problem that we are trying to help solve with many educational programs" and "is a problem of willingness." He was also critical of international organizations, such as the United Nations, that more often than not fail to actually maintain peace.

"Perhaps the United Nations should take a step forward by giving up the right to veto for some nations in the security council," he said. "I'm not a technician in this, but I feel like it is a possibility." He added that "it would be nice if everyone had the same rights."

For the time being, countries should negotiate to cease the production of weapons, he said.

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"The result of negotiation is to solve problems," said the pope. He cited the recent negotiations between Russia and Ukraine that resulted in a prisoner exchange.

While neither country agreed to stop producing or trading weapons, he said that this was a "step for peace" that was "positive."

It is "a hypocrisy," said Francis, for Christian, or Christian-influenced countries to engage in arms trade.

"It is an evangelical word, Jesus said it, somewhere in chapter 23 of Matthew. To end with that hypocrisy, and that a nation may have the courage to say 'I cannot speak of peace, because my economy earns so much with the manufacture of arms.'"

Violence, even as legitimate self-defense, should be a "last resort" said Francis.

"We are making an ethical progress, and I like to question all these things. It means that humanity goes ahead also for good, not just for evil."