UNITED NATIONS — The final preparatory meeting for next year’s review of the Nuclear Nonproliferation Treaty ended Friday with deep divisions, and U.S. Ambassador Robert Wood said reaching agreement at the 2020 conference “will be an incredibly difficult task.”

But he told the closing session of the two-week preparatory conference that “it is a task we cannot abandon.”

The NPT is the world’s single most important pact on nuclear arms, credited with preventing their spread to dozens of nations since entering into force in 1970.

It has succeeded in doing this via a grand global bargain: Nations without nuclear weapons committed not to acquire them; those with them committed to move toward their elimination; and all endorsed everyone’s right to develop peaceful nuclear energy.

Treaty members — every nation but India, Pakistan and North Korea who possess nuclear weapons, and Israel which is believed to be a nuclear power but has never acknowledged it — gather every five years to review how it’s working. They try to agree on new approaches to problems, not by updating the treaty which is difficult, but by trying to adopt a consensus final document calling for steps outside the treaty to advance its goals.

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