Barack Obama's bold pledge in Prague back in 2009 to realize a nuclear-free world resonated powerfully in Japan and culminated in his 2016 visit to Hiroshima.

There, the U.S. president said: "We come to mourn the dead, including over 100,000 Japanese men, women and children, thousands of Koreans, a dozen Americans held prisoner. Their souls speak to us. They ask us to look inward, to take stock of who we are and what we might become." In his view, Hiroshima represents an awakening to the need for a moral revolution.

But Obama also justified continued possession of nuclear weapons: "We may not be able to eliminate man's capacity to do evil, so nations and the alliances that we form must possess the means to defend ourselves. But among those nations like my own that hold nuclear stockpiles, we must have the courage to escape the logic of fear and pursue a world without them."