As President Barack Obama counts down his last months in office, he has returned to one of his earliest and most ambitious promises, one he fully articulated in Prague in 2009: "I state clearly and with conviction America's commitment to seek the peace and security of a world without nuclear weapons."

In the tumultuous years since, it's probably become clear to him the only way America can rid the world of nuclear weapons would involve the (until recently) unspeakable prospect of using them. Proposals for radical reductions have been largely stymied by political realities at home and growing threats abroad. The deal Obama did make with Iran infuriated Republicans (and some Democrats). Now he's going around the first branch again: He is asking the United Nations to help him revisit the Comprehensive Nuclear Test Ban Treaty, which turns 20 next month.