CONGRESS should remove the fence it had placed around the $1.5 billion needed to go into production of the second batch of 21 MX missiles. It is one of those close questions that turn out finally not to be a close question. By now, everyone knows the limitations of this 10-warhead machine. It is provocative and destabilizing: provocative because its high accuracy gives it a first-strike potential, destabilizing because its relative vulnerability might tempt a president to use it early in a crisis rather than risk losing it to a Soviet first strike. In a world where strategic decisions were based on qualities of weapons alone, this would be enough to doom the MX. But the real world is a different place.

The real world is a place where things have not only qualities but connections. The MX is connected in the first instance to the heavy Soviet missiles -- the 600-odd SS18s and 19s -- whose first- strike capability it is meant to match. Granted, the new weapons ordered up in the Ford and Carter years are now nearing deployment and so the moment is passing, but the fact is that at the moment the United States has no similar weapons.

The MX is further connected to the arms control talks that are to open in Geneva on March 12. A prime American purpose there is to induce the Kremlin to make deep reductions in the heavy missiles that have long given it, and it alone, a chance to knock out all of its adversary's land-based missiles in a surprise attack. This is the very definition of the Soviet threat. No one in his right mind can believe the Soviets would reduce these extra-threatening heavy missiles -- the numerical and political heart of their strategic force -- if they did not have to worry about a similar American threat. Ronald Reagan thinks this. Andrei Sakharov thinks it too. Absent a successful anti-missile defense, this is what deterrence is all about.

Mr. Reagan has been criticized for failing to keep arms-building and arms control in reasonable balance. Whether in his overall policy he is now heading toward such a balance is a fair question. On the MX, however, he has a record. Many times he has said the missile is essential for defense purposes. On July 16, 1983, he said: "If an agreement is reached which calls for deep reductions -- which, of course, is our goal -- the number of (MX) missiles could certainly be adjusted downward. . . . As opportunities permit, the U.S. position will continue to evolve." That statement reflects the obvious and sensible consideration that American forces are necessarily related to Soviet forces. A freshening of the 1983 statement would surely quash almost all residual congressional misgivings about the MX.