THE WAR IN Indochina, so often said by Americans to be "over," goes mercilessly on. In its latest episode, Vietnamese occupying forces in Cambodia, embarked on a seasonal offensive, have chased Cambodian guerrillas to their sanctuaries along the Cambodian-Thai border and driven some tens of thousands of Cambodian civilians from their miserable border camps into Thailand proper. Casualties among these civilians have been considerable. Thai troops, defending their sovereign territory, have fired back at the pressing Vietnamese. It is a nasty if all too familiar scene.

The friends of Thailand, including its noncommunist neighbors in Southeast Asia and the United States, have criticized Vietnam for its attacks on Cambodian civilians. Vietnam is the right target.Its ambitions for control throughout Indochina are the principal source of the region's continuing upheaval. The local government that Vietnam sustains in Phnom Penh could not possibly stay in power otherwise. Militarily, the Cambodian opposition to that regime appears persistent but a long way from success, or so its current difficulties along the remote Thai border suggests. Politically, it suffers from the inclusion in its ranks of the totally discredited Pol Pot communist forces, along with former head of state Prince Norodom Sihanouk and the anti-communist Son Sann. Vietnamese occupation of Cambodia remains, nonetheless, unacceptable.

Is any relief possible for poor Cambodia? If there is, it lies chiefly in new arrangements that might be made by the Soviet Union, patron of Hanoi, and the People's Republic of China, patron of the Pol Pot group. The two outside communist states are currently exploring a partial relaxation of the longstanding tensions between them. The United States gave up both its capacity and its taste to influence the outcome of the Indochina power struggle when it withdrew its forces and then its aid in the 1970s. Now the American government cannot do much more than offer occasional, largely gratuitous remarks from the sidelines. The fate of the region remains in others' hands.