The world took a big step toward a comprehensive test-ban treaty last week when China dropped its insistence on maintaining an option for "peaceful" nuclear explosions.
Beijing then promptly dampened some hopes by staging an underground blast at its Lop Nor nuclear test site over the weekend. While that deservedly touched off diplomatic protests and recriminations, it was quickly followed by assurances from China that it stood by its new commitment to a treaty.
More important, China announced that it would hold one more test before September and then stop its program. In this, the Chinese follow the French, who detonated a couple of final blasts at their South Pacific test site before shutting it down - amid even louder protests.
However questionable the rationale for such swan-song detonations, there are no questions about the value of a global ban on nuclear testing. It's a critical safeguard against renewed arms races and nuclear proliferation. And it's heartening that all five of the world's acknowledged nuclear powers - China, France, Britain, Russia, and the United States - have at long last agreed to such a pact, at least in principle.
Some rigorous negotiating remains. Verification is a sticky issue, with some countries, including China, resenting American insistence on using the most advanced means of detection - means not necessarily available to others.
Questions arise, too, over how many nations should constitute a quorum for implementing the treaty. Ideally, the initial signers would include not only the "key five" who have the bomb, but the "key three" - India, Pakistan, and Israel, who are known to have, or are close to having it - plus as many other nations as possible.
But the main task at hand is to complete the treaty, get it enacted this year through the General Assembly of the United Nations, and thus set a standard for the world. With the critical nations aboard, great pressure can be brought to bear on those who choose to violate the ban.
The clear national and international interest in avoiding future nuclear arms races should make the test- ban treaty a priority no matter who wins upcoming elections in the US, or in Russia.