IT'S a scenario to catch the eye of any Chinese military planner.
Sometime in the future, China gets into a dispute with an Asian neighbor, such as Japan, but finds the threat of its nuclear weapons made useless by a missile-defense system -- designed in part by the US.
China, in effect, would lose its nuclear option, a powerful ''card'' that helps ensure its influence in Asia. It would also be vulnerable to attack itself since it could not retaliate with its own nuclear arsenal.
Such scenarios have driven Beijing to join a chorus of complaints against a United States drive to use ballistic-missile defenses (BMD) to defend key military and urban targets in the Pacific region.
At the heart of the debate is whether the world's nuclear powers will continue to rely on ''mutually assured destruction,'' or MAD, a concept that assumes both sides will avoid war if there is the certainty of massive retaliation.
Critics say BMD, a ''star wars'' type system for regional defense, will bring instability. A nation might prefer to strike before a defense system is in place. Proponents argue that US-designed missile defenses in Asia are needed to prevent such ''rogue'' nations as North Korea from launching a nuclear attack.
The Pentagon indicated support for a BMD strategy in East Asia in a February report on the region, which referred to the possibility of developing a theater missile defense in cooperation with Japan. The US and Russia also agreed last week to consider expanding cooperative efforts in BMD-related research and technology.
Chinese reaction, however, has been overwhelmingly negative. Shortly after the release of the report, a Chinese foreign ministry official warned in Beijing that BMD could spur an arms race. ''If a country with nuclear weapons has a spear and then gets a shield, you can imagine what would happen,'' the official said. He added the US ''will increase the danger of nuclear war'' with BMD.
China is particularly concerned about the efficacy of its nuclear arsenal. ''This erodes their very small ballistic-missile force,'' says Paul Godwin, a professor at the National War College in Washington. China first exploded a nuclear device in 1964.
For BMD's supporters, China's concerns are overblown. Michael Green, a Japan expert at the Institute for Defense Analyses in Washington, says BMD is in China's interest, as it would keep the US engaged in Asia and Japan from developing its own nuclear force. And, he adds, ''the Pentagon is not looking to make China an enemy -- it wants to work positively to resolve issues.''
Others, however, are concerned about the reaction from a country that, despite US assurances to the contrary, often expresses the view that the US is practicing ''containment'' of China in the face of its growing military clout. China's defense spending has grown 40 percent in real terms over the last five years.
''Nothing much the US military can say will reassure the Chinese that the US isn't aiming to degrade the Chinese deterrent,'' says Iain Johnston, a professor at Harvard University in Cambridge, Mass., and an expert on Chinese foreign policy. ''There is an emerging consensus among Beijing hard-liners that US policy is to keep China weak.''
Mr. Johnston says that the Chinese are not mollified by those who say BMD is purely defensive and not targeted at eliminating China's nuclear deterrent. Even if the first generation of BMD technology cannot hit an incoming Chinese nuclear missile, he says, ''the Chinese view this as the thin edge of a wedge.'' And, he states, ''it encourages some in China who argue in favor of more missiles and warheads.''
In the past decade, prompted in part by the 1980s debate over President Reagan's pursuit of a US-based ''star wars'' defense system, Chinese nuclear strategists have taken a much closer look at ''limited'' nuclear-war fighting, or the ability to handle all levels of nuclear engagement, and to hit a wider range of possibly shielded targets.
A credible deterrent
Johnston points to trends in China's nuclear- force development as evidence: improved accuracy, consideration of tactical nuclear weapons, and multiple warheads on missiles.
While the Chinese force is minuscule -- about 300 long-range warheads -- as well as technologically behind US or Russian standards, it nevertheless has provided China with big-power status and a credible deterrent against other nuclear states. And with the US and Russia to reduce their nuclear arsenals by 2003 under the START II treaty, it will become propor-tionately more powerful.
Most analysts say that China will have to be part of any further arms-reductions agreements -- something that could be harder to achieve with BMD. ''If China gets too alarmed by this, it will affect their willingness to scale down,'' says Spurgeon Keeney, director of the Arms Control Association. ''It could have the effect of building a new technology race there.''
BMD could also reduce China's willingness to join in international nuclear accords. High on the wish list for those seeking greater Chinese participation in this arena are agreements on fissile materials production cutoffs as well as signing onto a comprehensive test-ban treaty, something China says it will do in 1996. Without China on board, says Lisbeth Gronlund, a senior arms analyst at the Union of Concerned Scientists in Cambridge, Mass., there is little hope for signing up other Asian nations.
''The US has decided one of its big goals is to pull India and Pakistan into some sort of regime,'' Ms. Gronlund says. ''China is key to this.''
Some analysts argue that greater communication between the China and the US can bridge many of the difficulties posed by US support for BMD. The Institute for Defense Analyses' Mr. Green says that improvements in military-to-military dialogue are a necessary first step.