ONE CURRENT in the Reagan administration sees arms control as an imperfect prop of American security but one politically and strategically worth trying to strengthen. A second current considers it a menace to American security -- because the Kremlin can cheat and because the arms control "process" invites the Soviets to play on American hopes for relaxation and thereby keeps the United States from doing what it otherwise would do in defense. The civilian leadership of the Pentagon represents this second tendency and, in the continuing internal battle, it has just won an extremely important round. It is not a complete rout for those in the administration who accept arms control as a legitimate enterprise and as a central element in a mature Soviet-American relationship, but it throws them back hard.

SALT II, the arms control treaty negotiated by Jimmy Carter but never ratified, is the battlefield. Candidate Reagan denounced it, but President Reagan decided not to underout it, finding it both useful (it has required the Russians to dismantle large numbers of weapons to stay under SALT ceilings) and in a sense painless (it has not kept the United States from pursuing any necessary strategic programs). But the very idea of SALT offends some hardliners. Allegations that Moscow is cheating, continuing its buildup and stonewalling at Geneva are the familiar features of their attack.

The Soviet record on compliance troubles even ardent advocates of arms control. Still, the administration itself tells Congress that in some though far from all areas, Soviet compliance has improved in the last year. The president says carefully there has been no "real" progress, acknowledging that it's a judgment call. It is enough of a judgment call, in fact, to lead the more moderate conservatives to urge that any American responses to Soviet violations be kept within the SALT treaty.

Precisely here lies the significance of what the president has just done. He is going to dismantle two old subs, as a new one comes into the fleet, to stay under a SALT ceiling on multiple-warhead missiles. But he's doing it, he insists, on military and economic grounds, and in the future he'll base strategic decisions not on SALT standards but on "the nature and magnitude of the [Soviet] threat." So it becomes possible that at the next moment of truth -- in the fall, in respect to bombers carrying cruise missiles -- he will for the first time openly "break out" of SALT. The United States is "no longer bound" to continue observing SALT II, Defense Secretary Caspar Weinberger says. "It's very simple."

It's a mistake. The Joint Chiefs of Staff, in the past anyway, have believed that Moscow is far better placed than Washington to field new weapons quickly if the two sides break out; that's why they have supported the treaty. The Soviet leadership must deal with a tight budget, but not with an independent Congress, opposition and public. The notion that Moscow, under this sort of American pressure, will easily change the ways of which the United States complains is, at the least, highly speculative. Is the United States "no longer bound" to SALT II? It had better turn out not to be so.