LAST SUMMER GEORGE MCGOVERN, by way of lamenting Cambodia's assaults on the lives and rights of its own citizens, said he hoped some "military force" would "knock this regime out of power." Over the weekend a powerful army from Vietnam did just that. The attack was not, of course, a direct response to Sen. McGovern but it probably did arise in part from a perception that Pol Pot's demise would be accepted as a deliverance in every corner of the international community except Peking. Washington, for one, waited two weeks until the invasion had succeeded before criticizing it. Invasion is not the answer to every "Idi Amin" -- the current prototype of the deservedly despised political leader who invokes sovereignty to cloak misrule. But when it happens there is no point being sanctimonious.

The nature of the regime in Phnom Penh affected more than the international climate. The invading Vietnamese, with their thin cover of Cambodian insurgents, sliced like a knife through butter. The people in the countryside could hardly have welcomed "liberation" by their historic adversaries. But their patriotism may have been blurred by relief at their rescue from a hated regime. Ambivalence of this sort is suggested by the performance of Prince Sihanouk. No sooner had the former leader been released by Pol Pot after almost four years' imprisonment to plead Cambodia's case at the United Nations than he denounced Pol Pot and said he might not go to the U.N. after all.

The one-month-old Cambodian group created -- and now seated in Phnom Penh -- by the Vietnamese at once announced it would restore the religious rights, public services and right of urban residence arbitrarily terminated by Pol Pot. Whether the Pol Pot guerrilla fish can find enough of a popular sea to sustain a resistance is uncertain.

The administration watches nervously from the sidelines, hoping at the least that China and Russia won't be drawn directly in. Its restraint is appropriate, if only on historical grounds. A case can be made that it was the United States' own incursion in 1970 that upset the political balance, such as it was, and set in motion irreversibly the turmoil and tragedy that have been Cambodia's lot ever since. Certainly no country has less standing to inveigh against the Cambodian incursion of another.Yet China in particular, whose client in Cambodia has been humiliated, surely is smarting -- and calculating the odds on either outsting the new group in Phnom Penh or retaliating against Vietnam, which is a client in turn of China's own archrival in Moscow. It is a clouded scene and the United States, apart from urging caution, has little choice but to wait and see how it clears.