With as many as 120 warheads, Pakistan could in a decade become the world’s third-ranked nuclear power, behind the United States and Russia, but ahead of China, France and Britain. Its arsenal is growing faster than any other country’s, and it has become even more lethal in recent years with the addition of small tactical nuclear weapons that can hit India and longer-range nuclear missiles that can reach farther.

These are unsettling truths. The fact that Pakistan is also home to a slew of extremist groups, some of which are backed by a paranoid security establishment obsessed with India, only adds to the dangers it presents for South Asia and, indeed, the entire world.

Persuading Pakistan to rein in its nuclear weapons program should be an international priority. The major world powers spent two years negotiating an agreement to restrain the nuclear ambitions of Iran, which doesn’t have a single nuclear weapon. Yet there has been no comparable investment of effort in Pakistan, which, along with India, has so far refused to consider any limits at all.

The Obama administration has begun to address this complicated issue with greater urgency and imagination, even though the odds of success seem small. The recent meeting at the White House on Oct. 22 between President Obama and Prime Minister Nawaz Sharif of Pakistan appears to have gone nowhere. Yet it would be wrong not to keep trying, especially at a time of heightened tensions between Pakistan and India over Kashmir and terrorism.