GREAT TERROR overtook Cambodia when the United States departed in 1975. The Khmer Rouge killed a million or two of their fellow citizens, and subsequently the Vietnamese invaded. Resistance goes on. The United States has had no taste for any sort of military involvement in any part of Indochina in the last 10 years, but the issue is now arising of whether Washington should not go beyond political support and extend to the noncommunist Cambodian resistance -- to Son Sann's Khmer People's National Liberation Front, not to the communist Khmer Rouge -- modest aid, through the Thais. This is the aim of a $5 million aid proposal, launched in the House Foreign Affairs Asian subcommittee by its chairman, Rep. Stephen Solarz (D-N.Y.), and moving along now.

What? Aid to Cambodia? Aid that could take a military form? A new American involvement in Indochina? The very thought is enough to send a surge of alarm through the large community of Americans who have come to identify Indochina as the ultimate American disaster area in this century. That the contemplated aid is meant to be small and indirect, with no physical American participation on the ground, is sure to be taken by many people as a sign either that it contains a core element of deception or that it will inevitably grow and slip out of control. It arises during the 10th-anniversary review of the American defeat, which will no doubt intensify the warnings against it.

Is it not time, however, to take a more measured view? The so-called Vietnam syndrome has been operative for a decade: It tends to make Americans shrink from uncertain involvements where force may have to be brought to bear. We have never felt that there was a positive value in the United States' making a clear demonstration that it was no longer hobbled by an excessive regard for what some see as an increasingly irrelevant and even dangerous "lesson" of the past. On the contrary, the leading Vietnam lesson -- the requirement for the utmost care and seriousness in defining and defending American interests -- remains central. But, of course, the world is still there, and the United States still feels itself to be a global power. In all of this, bringing force to bear, or supporting the use of force by others, has its irreducible role.

In Cambodia? It meets certain of the agreed tests: The friendly party -- the noncommunist KPNLF -- is reasonably democratic. The hostile party is not a sitting government of some legitimacy but a foreign occupying power -- the Vietnamese. Americans care about the victims -- Cambodians. Aid could serve the practical purpose of helping ensure the KPNLF a seat at an eventual settlement table. There is an American strategic interest -- to contain Soviet power and reassure American friends. The possibility of a supportive political consensus is at least suggested by the fact that the sponsor, Rep. Solarz, was first elected to Congress as a liberal Democrat 10 years ago just as South Vietnam and Cambodia were falling under communist rule.

You might think the Reagan administration, devoted as it is to an ideology of freedom, would be out front on this one. Actually, it's hanging back, letting others break the path. It feels that the case has not been made that noncommunist Cambodians need American aid or would profit from a more direct American involvement, and that providing such help, even in a token way, might put a distracting American stamp on a cause whose Asian sponsorship is an important asset. American policy in Cambodia, under the last two presidents, has sought out a useful supporting role. That some Americans are prepared to consider going beyond that marks an event in the evolution of the political culture. But the burden remains on those who would take the step to construct the necessary doubly persuasive justification for it.