THE UNITED STATES has reversed not so much its Cambodia policy as its Vietnam policy, though the first benefits may fall to Cambodia. This far larger result is clear from Secretary of State Baker's announcement that Washington, which has striven to isolate Vietnam for the 15 years since the American defeat, will now address Hanoi directly on the subject of Cambodia. Instead of stiff-arming Hanoi and its client Hun Sen regime in Phnom Penh, the Bush administration now tentatively takes them as partners in the urgent project of keeping the Khmer Rouge from returning to power.

Extending a hand to Communist Vietnam, inviting its cooperation to meet a rival Communist group's challenge to a Communist regime in Cambodia, becoming ready to help a former enemy come in out of the international cold: all this constitutes a major development in American politics as well as diplomacy. Certainly it was no easy turn for an administration holding that the Vietnam War was a noble cause and the war's loss a calamity. Loyal to this tradition and its constituency, President Bush had subordinated accommodation and reconciliation with Vietnam to difficult policy changes on the part of Hanoi. He had resisted his critics' pleas to make Vietnam and Hun Sen, himself a former Khmer Rouge, part of the solution to the reemerging crisis in Cambodia. Instead, he had focused on backing the two small non-Communist groups fighting Hun Sen, thereby unintentionally but unavoidably helping the Khmer Rouge.

Seeing the genocidal Khmer Rouge move ahead on the battlefield and his policy critics move ahead in Congress, however, Mr. Bush made his move to Hanoi.

Two sets of parties must be brought along to save Cambodia. The ''Perm Five'' -- the five permanent big-power members of the Security Council -- have been working up a plan to transfer Cambodian sovereignty temporarily to a United Nations structure that would run elections. It is encouraging that Beijing, patron of the Khmer Rouge, and Moscow, patron of Hanoi, are in rare semi-harness and that Japan is applying its economic weight. Still, the four Cambodian parties -- Hun Sen, the two non-Communist factions, the Khmer Rouge -- must also cooperate, or the first three of them must find a way to neutralize the fourth.

President Bush's new Vietnam policy improves the American positioning -- not only tactically but morally -- in the difficult maneuvering over Cambodia. Regardless of whether a solution there is reached, it puts the United States in a position in Vietnam to set the Vietnam War behind and to start applying in Southeast Asia some of the policies ''beyond containment'' that are transforming Europe. Mr. Bush is now on the right track and an immensely important one.