THE MOST CRITICAL military, political and economic changes in Asia since those that followed the Vietnam War could be coming to the region.

As the economic crisis continues to weaken all Asian governments, the Indian nuclear tests put at risk what there is of a strategic balance. The possibility of major political change in Indonesia only adds to the uncertainty in a zone that has often seemed characterized by immobility.The country to react with the greatest concern to the Indian tests is the United States, but China holds the key to the new strategic situation in Asia. The Indian nuclear tests are also a test of Chinese political maturity. Some believe China has passed one such test this year, in not reacting to the economic troubles in the region by attempts, by devaluation or other means, to undercut competitors in export markets.

Whatever the truth of that, China is the only country able to stop the arms race between Pakistan and India leaping to a new level and the only country that can persuade New Delhi to follow the Chinese example and sign the Comprehensive Test Ban Treaty.

But if it is to succeed, China will have to greatly modify policies it has been following for years. Those policies have offered military support, including nuclear and missile technology help, to countries in Western Asia, including Pakistan, Iran and, previously, Iraq.

The Chinese have thus contributed to developments that could produce usable nuclear weapons in the hands of several Muslim states. Pakistan and Iran, at least, have cooperated with each other on nuclear programs and missile technology. The countries against which such weapons might be used are, in the first case, Israel and India.

If serious sanctions are brought in, allowing the Bharatiya Janata Party to pose as defender of an embattled nation, and if China makes no move to meet Indian requirements, the danger is the BJP will continue with an active nuclear policy because it is such a domestic political asset, rather than following the Chinese example of testing and then signing the test ban treaty.

It is almost certain to be impossible to prevent a Pakistani test series, but after that both countries may follow the "test-and-sign" strategy. The senior Pakistani nuclear scientist who said he and his team were now in the position of cooks in the kitchen waiting for the order reflected the reality. Pakistani pride, unfortunately, will demand tests.

But weapons development in Pakistan is dependent to some extent on Chinese technical support and to an even greater extent on the general relationship with that country, which has at times of danger become a quasi-alliance. China's influence with Pakistan should be sufficient to produce a test-and-sign strategy in Islamabad, if it is matched in New Delhi.

The now-established traffic between China and the west Asian Muslim countries to which it gives military help has to end or at least be limited and become transparent. This is the key shift in Asian security that could head off a South Asian proliferation and arms race.

This would not be easy for China, nor is it uncomplicated, since the other main country concerned is Iran, with which Russia also has military technical relations. In addition, Beijing has to convince India it is ready to be accommodating on border questions. That means shifting from the Chinese position that the present line of actual control, which in most cases favors China, should be the basis of negotiations.

Ironically, American hopes that China will join the Missile Technology Control Regime, cutting out mis-sile support to Pakistan and Iran, may have been one reason why India chose to test.

Achieving the maximum level of technical nuclear proficiency before accepting controls, thus freezing the hierarchy of nuclear capacity at the moment most advantageous to a par-ticular country or group of countries, is an old story in arms control. It certainly motivated France and China before they signed the CTBT, and the Indians would claim it motivated all the established nuclear powers in their arms control maneuvers.

However, it may represent the best way out of the potential South Asian crisis, if China and America are ready to lead the way.