IS IT NOT astonishing that the Soviet Union should now be pondering whether to invade a second neighbor, neither of which committed a single hostile act against it -- this one not merely a friend but also an ally -- within a year's time? Or is it so astonishing after all? It is tempting to imagine that what we are seeing is an otherwise matured great power taking a regrettable but aberrational detour from detent. But two unprovoked aggressions in a year -- what can that mean but that the Soviet Union is immature and unmellowed to the point of being a menace to international peace? By all conventional measures, Soviet power has grown impressively in recent years. Yet the more reason it has to feel secure and to countenance a bit of turbulence on its borders, the more it seems determined to flex its muscles, to crush innocents, to throw its crude weight around.

It is suggested that the last thing Moscow wants to do is to invade Poland. Amendment: the last thing Moscow wants is to lose Poland. To say even that, however, is to give the Soviet Union much more license to savage its neighbors than law and minimal civility allow. There is a point at which efforts to anticipate and "explain" an invasion end up justifying it. In Poland, for instance, where the Communist Party has never ruled by any means but force, the party's monopoly on power is now under challenge by a genuine popular movement. But that the mighty Soviets quiver pathetically in fear of the very thought of it confers on them no "right" to undo it by force. Similarly, it is quite so that Poland is a vital defense corridor for the Soviet Union. But where is the threat?Surely a phantom paranoid suspicion does not qualify as grounds for the rape of another country's sovereignty.

Outsiders hoping to deter an onslaught have limited options. They can urge restraint upon the Polish parties, as unfair as it is to put the onus upon the innocent likely victim. They can seek by a sort of political body English to persuade the Russians that their best course is to let the Poles sort out their own affairs, as the Poles give full evidence of being prepared to do. They can remind Moscow of the costs it could expect to incur if it invaded. The Carter administration's reminders have been strong. But since Mr. Carter is a lame duck, the incoming administration's reminder is more to the point. The consequences of an invasion, said Richard Allen, a top Reagan aide, would be "severe and long-lasting. It would border on wrecking relations for a long time." Who doubts it?