On Monday, representatives from 188 nations gathered in New York for a month-long review conference of the Nuclear Non-Proliferation Treaty (NPT). Since it went into effect in 1970, the NPT has been a powerful testament to how deeply an accord can shape the world.
The treaty has made important progress. It has prevented the spread of nuclear weapons to more countries - a dire prediction in the early 1960s said perhaps 20 or more states would have nukes by 1970.
It helped persuade countries such as Argentina and Brazil to halt their nuclear weapons programme and secured the accession of Belarus, Ukraine and Kazakhstan as non-nuclear weapons states after the break-up of the Soviet Union.
The NPT was and remains a political grand bargain. At the conclusion of the negotiations in 1968, the signatories agreed that in exchange for forsaking nuclear weapons, states would be entitled to receive, acquire and develop nuclear technologies for peaceful use. The nuclear weapons states - the five countries that had detonated nuclear explosives before 1967 (Britain, China, France, the Soviet Union and the US) - would pledge to undertake nuclear disarmament in good faith with the goal of eventually eliminating them.
But the treaty has been under increasing strain since the late 1990s. It has yet to achieve universality as India, Israel and Pakistan, the three de facto nuclear weapons states, remain outside the regime.
That goal became more elusive with North Korea's withdrawal in early 2003 - the first country to do so - and concerns that others may follow. Members are free to withdraw from the treaty by giving 90-days' notice.
Indeed, North Korea's open defiance of the international non-proliferation regime - and its claim to have developed nuclear weapons - pose a serious threat to regional and international security. Pyongyang apparently test-fired a missile on Sunday, and US intelligence suggests it may have learned to arm its missiles with nuclear warheads.