CHINA'S ADVANCE into Vietnam is no less alarming for having been no surprise. The Chinese, who set great store by credibility, had warned that they could not accept Vietnam's recent takeover of Cambodia, coming as it did on top of several years of what Peking regards as systematic border provocations by Hanoi. So over the weekend, saying they did not covet "a single inch" of Vietnamese territory, the Chinese "counterattacked" -- and simultaneously proposed border negotiations. Though Peking did not say so, it is widely assumed that China will also demand that Hanoi withdraw its forces from Cambodia.

The urgent question is whether the Kremlin will respond with force to this attack by its arch-rival, China, on its friend and treaty partner, Vietnam. It seems indicative, though not conclusive, that in its first verbal reaction yesterday the Soviet Union -- while insisting that it would "honor its obligations" to Hanoi -- cautioned Peking "to stop before it is too late." That is, it is not yet "too late" -- if Peking confines itself to teaching the "lesson" that it had previously said was due, rather than claiming Vietnamese territory or threatening to topple the Hanoi regime. China's own warning to Hanoi "to stop on the precipice" bespeaks a similar concern to limit the scope of the conflict.

The Carter administration, aware that sooner or later a Chinese attack was coming, appears to have positioned itself in two ways. To ward off the expected Soviet charge that it had conspired with Peking, the administration spoke out strongly against an incursion before it took place and, in the event, immediately reinforced the Soviet demand that Chinese troops be withdrawn. To keep faith with the People's Republic, the administration let it be known in advance that a Chinese response would not upset the new Sino-American connection; after the attack, officials warned the Soviets not to retaliate against China and, even while demanding that China quit Vietnam, demanded that Vietnamese troops leave Cambodia. But whether these formulations will serve their intended purpose of limiting hostilities and keeping American diplomacy on its several tracks is uncertain. The key decisions are in others' hands.

Just the other day Hanoi rejected a Security Council appeal to withdraw its forces from Cambodia. Now it asks council help in prying Chinese forces off its own territory. Two interventions need to be ended; that may be the deal to be made. But since Vietnam, in Cambodia, not only invaded but changed the local regime, it will be a hassle to arrange the terms on which the two invaders' forces are withdrawn. That the former Cambodian regime was a monstrous one, and even now seeks to return to power, unquestionably complicates the problem.

Any diplomatic effort launched at the United Nations will necessarily involve a great deal of jostling by Moscow and Peking and, in a quieter way, Washington. The current war and tension are proof enough that none of the three has conducted a very sensible policy in Southeast Asia in recent years. But they all have a contribution to make if the strife and suffering there are to be reduced, and it is to figure out how to establish a more stable regional balance of power.