PARIS -- The Khmer Rouge are poised to take over, and the Vietnamese army is not there to stop them. With one policy statement last week, Secretary of State James Baker changed the rules of Cambodia's year-old civil war and positioned the United States to become the chief broker of peace for that country.

America's long-overdue decision to punish rather than appease the Khmer Rouge set in motion a series of adjustments that overnight give hope that Cambodia will be saved from a second bloodbath at the hands of Pol Pot's soldiers. By removing American support for the Khmer Rouge coalition, which has held the Cambodian seat at the United Nations, Baker transformed the Khmer Rouge from legitimate representatives of the Cambodian people into international pariahs. By opening talks with Vietnam and acknowledging for the first time that the Vietnamese have withdrawn from Cambodia, he transformed Vietnam from the chief enemy into a legitimate partner in the search for peace.

So much for all the handwringing about American weakness in Indochina. Baker's initiative highlights instead how the United States has won control over the Cambodia negotiations. The United States has determined which Cambodian groups are to be rewarded and -- more important -- which are to be punished.

Baker decided to wrest control of the negotiations after a French-sponsored conference on Cambodia failed here last August. Vietnam withdrew its remaining occupation troops in September, ending a decade of military occupation and removing any obvious rationale for supporting the Khmer Rouge coalition -- which includes two very junior non-Communist groups -- as a force resisting the Vietnamese occupation.

Behind the scenes, Washington strengthened its support both for the resistance coalition and for sanctions against Vietnam and the Phnom Penh government that Hanoi had originally installed. Saying Vietnam should not be rewarded for its "unilateral withdrawal," Baker intervened against Hanoi at the World Bank and ensured that the Khmer Rouge coalition retained Cambodia's U.N. seat.

The Americans demanded essentially that the Phnom Penh government dissolve itself and hand over much of its power to the Khmer Rouge coalition or face the consequences on the battlefield. Civil war ensued.

Washington tightened its blanket sanctions against the Phnom Penh government and dissuaded American allies from aiding or even visiting Cambodia.

Baker modified his ideas in November and announced a plan to find a "minimum role" for the Khmer Rouge in a temporary government pending free elections. He persuaded the U.N. Security Council to convene fresh talks on Cambodia. There American diplomats clung to the unusual formula demanding an all-or-nothing peace solution. They publicly declared there could be no cease-fire or halt in arms shipments to the Cambodian armies until a comprehensive solution was approved by all parties, including the Khmer Rouge.

Starved by the all-inclusive U.S. sanctions and pushed by its Soviet patron, the Phnom Penh government gave in to nearly every demand, including accepting free, democratic, U.N.-supervised elections.

But this was not enough for the ungrateful Khmer Rouge. They demanded an equal -- not a minimal -- role in any government and, through their Chinese patrons, they got the U.N. body to erase any reference to the genocidal nature of their four-year rule, which had caused the death of more than a million people and destroyed the society. They sabotaged every hopeful step toward peace. Like the Nazis to whom they are so often compared, the Khmer Rouge used the time gained by these exercises in appeasement to position their forces around the countryside in preparation for military conquest.

Baker's abrupt policy shift is a belated admission that the Khmer Rouge have been the chief benefactors of this past year of negotiations. But there are other reasons for the change: to cooperate with the Soviet Union, which has cooperated with the United States on other global issues; to head off a congressional revolt that would upset Republican electoral strategies this fall; and to quiet the steady stream of criticism in the American press.

Now Baker must address the central issue he and the rest of the international community have tried to avoid all along: how to stop the Khmer Rouge army. Despite the American attempt to blame Vietnam for the Cambodian war this past year, the stark reality is that the Khmer Rouge army is poised to take over Cambodia again, and the Vietnamese army is no longer around to stop it.

Baker's decision to talk to Vietnam shows his seriousness. If he matches it by coordinating a plan with Vietnam, Thailand and the Phnom Penh government, he could block the Khmer Rouge and bring an end to a haunting relic of the Cold War.

The writer is a special correspondent for The Post