Perhaps no internal struggle within the Reagan administration has been so protracted and painful as that centering on SALT II. It has been a battle not so much over the value of the treaty's terms as over the validity of trying to negotiate any terms at all with an adversary who is untrustworthy. One part of the administration finds the idea of negotiating nuclear checks and balances a dangerous illusion. Another finds SALT II a run-down but still convenient half- way house on the way to sturdier controls. No resolution is in sight.

The latest issue was whether to retire two old submarines in order to introduce a new one without breaking a key SALT subceiling on multiple-warhead missiles. Does it seem simple? Not for a government nagged by the thought that the Soviets are constantly cheating. In fact, according to one expert observer, Sen. Sam Nunn (D-Ga.), the Soviet record is mixed. "Congress has been informed by your administration," he told the president, that in some areas the Soviets have either "remained in compliance or improved" in the last year and in others there are "clear violations." He recommended a package of "proportionate responses" -- the president's phrase -- all consistent with SALT.

The president, on the submarine issue, did the right thing by accepting the State Department's recommendation to dismantle the two old subs and to stay under the SALT II ceiling. Pentagon civilians had urged keeping the subs around. This would have been the first open treaty break by either side. As its partisans intended, it would have called into fundamental question the American attitude toward arms control. It would have been a mistake.

At the same time, however, President Reagan took a long step in the Pentagon's direction. In language yet to be made public, he approved a formal change from a policy of not undercutting SALT II if the Soviets show matching restraint, to one of readiness to abandon SALT II, if there is a military reason, unless the Soviets otherwise satisfy American requirements. Officials warn not to read the wrong message into the decision to dismantle two old subs that require expensive overhaul and then would have few years of life left. There will be much greater military reason to break the next SALT subceiling that looms -- the subceiling on bombers carrying cruise missiles.

That decision is a few months away, and it implies a drama later in 1986. Will President Reagan, with two years remaining, break SALT II and thereby limit chances for the broad settlement he insists he seeks with the Kremlin? Stay tuned.