Back to basics: would the United States be justified in invading Nicaragua or otherwise using direct force against it? Events have moved the question a nervous distance from the abstract and propagandistic toward the real.

It's a harder question than Grenada, where special factors -- a clutch of possibly endangered American citizens, an unrigged appeal to intervene from democratic neighbors, a palpable collapse of local order -- added weight to a generalized fear that the Soviet Union might somehow gain strategic advantage in a region that most of us regard as an American sphere of influence. Though that same generalized fear is operative in respect to Nicaragua, no similar special factors hold.

But what of this generalized fear? It has at least two elements. First, suppose Nicaragua threatens its neighbors. This is more than conservative fantasy. The embers were glowing in El Salvador at the end of the 1970s but, with Cuba, Nicaragua supplied the fuel and the bellows that turned them into fire -- in the name of revolution.

The opportunity for the United States to respond to that offense against conventional order, however, passed. The Sandinistas' protestations that they haven't done much lately stand essentially unrebutted by the administration.

Apparently realizing how weak is its proof that Nicaragua currently aids guerrillas across its borders, the Reagan team has turned to emphasizing the capacity the Sandinistas' arms give them to launch an invasion with their own forces. But I find no one outside the team who believes that to be a serious possibility. The Reaganites look mean and silly when they brush past the possibility that one reason Nicaragua is amassing arms is to defend against possible American attack.

You could say the Nicaraguans are being either dense or disingenuous in failing to realize that a fear of casualties and a desire to make all military engagements short and sweet still hobble American military policy; that not even a reelected Ronald Reagan has a free hand. I have said and thought these things. But aerial or naval bombardments are in another category. It remains understandable that the Sandinistas, who keenly recall previous American interventions and who feel the administration's hostile pressures, are taking prudent steps to strengthen their forces -- and to assert innocent intent.

A new element has now snapped into place in Washington: the feeling that not only a Nicaraguan assault on a neighbor but the mere act of receiving certain arms -- in this case MiG21s -- would trigger justifiable American military action. This position was asserted by the administration and accepted without discussion by many policy critics. What about it?

This is the point at which considerations of Nicaragua's sovereign right of self-defense start bumping against a legitimate American concern not to see Nicaragua becoming, by progressively darker shades of gray, a Soviet base or strategic outpost. It is the old and familiar Cuba question, which the Soviet-American agreement ending the 1962 missile crisis never conclusively answered.

It is not that the arms which American officials portray as a threat to other Central American states would be the slightest match for the firepower the United States could easily bring to bear. Managua's possession of such arms does not raise any new threat to the territorial United States or change materially the regional balance of power.

Still, the spectacle of political change being encouraged and propelled by the engine of Soviet military power is disturbing. There is such a thing as the American back yard, even though we have not always acted responsibly in it. In the final analysis, it is the Soviet military connection to the region that makes us uneasy. Even those of us who feel we can live with the Nicaraguan revolution do not want that connection strengthened.

Here is the heart of it. I simply do not feel the Reagan administration has made the energetic good-faith effort it should have made to live with the Nicaraguan revolution and to diminish its excesses, dangers and fears. The question of using force arises not out of the intrinsic nature of the Sandinistas, as the administration essentially claims, but out of a set of tensions to which the administration has made its own rich contribution -- by sponsoring an armed insurgency of uncertain objective when political options had not been fully explored; by failing to do all it could to get the Nicaraguan elections on track; by not making serious use of the Contadora process.

I resent being confronted with the military question when the administration evades the political question. Back to basics: perhaps there's still time.