THE REPORT and documentation made public by the State Department yesterday nail down the foreign Communist role, especially Cuba's, in arming, training and directing the insurgency in El Salvador and in supporting it with a global propaganda campaign. This had already become apparent to the Carter administration, which on that basis resumed the supply of arms to the Salvadoran government in January. Mr. Reagan needed no further convincing but he did see reason to make a case on which to build support for his policy. So for those persons and foreign governments in need of hard evidence to convince themselves or their publics of Communist interference, here it is in unprecedentedly detailed, comprehensive and timely form.

The State Department paper does not assert that foreign Communists started the trouble in El Salvador -- the trouble is attributed to persisting misrule and a tradition of violence -- but that they "intensified and widened" it, especially starting last fall. This seems to us a fair assessment. Necessarily, it leaves open the question of whether the insurgency can be sustained without external support. Events may soon supply an answer. Even with substantial foreign aid, the guerrillas' "final offensive" flopped.The administration reports, cautiously, initial success in inducing Nicaragua to halt the further flow of arms from Cuba. The civilian president of the Salvadoran junta says that, if the flow does not grow, the army can handle the guerrillas. Secretary of State Alexander Haig says, "The [military] situation is under reasonable control."

If it is, the full spotlight will be on the junta. Here the signals coming from the administration will be critical in determining whether the junta's reform wing or its repression wing advances. So far, those signals are mixed. The administration is putting up more money for reforms, but Mr. Haig, saying that "all reasonable steps have been taken," seems disinclined to press the junta to control errant soldiers and the allied death squads. Stirrings of Salvadoran interest in talks aimed at establishing a broad democratic base do not seem to be getting American encouragement. It is possible -- it would be tragic -- for the junta to hold its own in the battle against the guerrillas and lose the war for justice and popular rule.

The administration's attitude to this prospect is unclear. In looking at El Salvador, it has a glint in its eye -- the prospect of cutting Fidel Castro down to size. Recent statements by Mr. Haig and by presidential counselor Edwin Meese III pin responsibility on Cuba and lay out a rationale for treating "the problem of Cuba" at the source" by any means necessary, including military. By "the problem of Cuba," it is evident, the administration has in mind the Cuban role -- and to an unspecified extent the Soviet role -- in places far from Central America. This is what lies behind the extraordinary, and otherwise inexplicable, attempt the administration has been making to line up allies and friendly governments behind its approach to miserable little El Salvador.