The widespread relief and even rejoicing in this country that have greeted the Bush administration's change of diplomatic signals on Cambodia cannot be allowed to obscure that the hard core of the Cambodian problem -- preventing a Khmer Rouge military takeover -- remains undented and intact.

The fact is that the question of which Cambodians take over their country's seat in the United Nations is mostly symbolic. Which Cambodians take over their country's government is what counts. The change of signals on the one is useful to the extent that it expedites ongoing United Nations diplomacy centered on the other.

The expressed relief, or much of it, may have had less to do with new prospects in Cambodia than with the feeling that it is past time to set matters straight with Vietnam. Call it realism, liberal guilt or post-Cold War review. It is time. But because the Khmer Rouge pose a raw threat, the impact on Cambodia requires first concern.

The United States bowed to this priority. From using the Khmer Rouge (indirectly, through support of their coalition with the non-Communist resistance) against the Hanoi-seated Hun Sen in Cambodia, it now begins using Hun Sen and Hanoi against the Khmer Rouge. This responds, however tardily and incompletely, to the crisis created by the Khmer Rouge onslaught.

The United States' Southeast Asian friends, in complaining about the American initiative, seem all too ready to sacrifice Cambodia to their longer-range fear of Vietnam. After all, it is as a result of the policy that these governments have favored that Cambodia again faces a Khmer Rouge takeover.

These friends make another point, however; so does Sen. Bob Kerrey (D-Neb.). The American opening to Hanoi gives the Khmer Rouge their own opening to deepen their political appeal in Cambodia by evoking the current of nationalist, anti-Vietnamese sentiment that runs there. This is the downside of the American attempt to isolate the Khmer Rouge.

How, then, to move on?

First, it is foolish to think of winning a war against the powerful, Chinese-armed Khmer Rouge. The likeliest force to deploy against them would be the Vietnamese, and this is everywhere unacceptable. Kerrey is on the mark in contending that the Khmer Rouge are the surest to profit from continued fighting. It is urgent to shift the struggle from the military to the political realm.

But this cannot be done simply by summoning a conference of Cambodians, and least of all a conference of Cambodians without the Khmer Rouge, as some suggest. Included, their commitment to a common solution is doubtful but perhaps conceivable. Excluded, their grim energy would inevitably go to spoiling the outcome, as the other Southeast Asian nations duly warn.

To get all the Cambodians in a room, however, outsiders must arrange the furniture. This is what the ''Perm Five'' -- the five permanent members of the United Nations Security Council -- have been lurching toward. Their success would stop the fighting and produce elections.

The Soviets, for their own reasons, seem to be terminating a previously open-ended commitment to their old clients in Hanoi. Not unreasonably, this has raised fears that China will be tempted to keep its Khmer Rouge guerrilla clients in the battlefield. It falls to the United States and others, including the Japanese and Southeast Asians, to induce China to bring the Khmer Rouge into line -- by aiming for a geopolitical equilibrium in tippy Southeast Asia and by manipulating economic carrots.

In short, it is all very well to salute a change of American policy that distances us from the Khmer Rouge and moves toward accommodation with Vietnam -- although here it must be said that Vietnam's continuing human rights cruelties, as documented this month to the United Nations Human Rights Commission, cast a pall on the idea of reconciliation.

But the American change has value to Cambodia only as part of the international effort to weave a safety net under the torn and falling Cambodian nation. A gambit that unhorsed the Khmer Rouge in New York but failed to keep them from power in Phnom Penh would be an obscenity. Nor is it beyond imagining.