NATO MAY BE one of the few measurable national security successes of the Reagan period.

Final results aren't in, but enough progress has been recorded for the man who counts most in NATO affairs these days, Sen. Sam Nunn (D-Ga.), to let the Alliance slip a key deadline that he and Sen. William Roth (R-Del.), set for it last year.

At that time they put in an amendment requiring NATO's European members to improve promptly the Alliance's capacity to meet a conventional attack; otherwise, the United States would start reducing its troop levels. The amendment lost, barely, but NATO got the message. Its new secretary general, Lord Carrington, helped convince members that what they first received as a threatening and intrusive American diktat was actually a great policy ounity. The Alliance went on to make the requisite commitments regarding munitions stocks ("sustainability") and reinforcement facilities ("infrastructure"). As a result, Sen. Nunn says it has earned another year to show it can deliver on those commitments.

Look closely here. Sen. Nunn and his policy comrades, NATO military commander Bernard Rogers and U. S. Ambassador to NATO David Abshire, are not talking just about hardware. Nor are they talking just about the familiar grinding NATO argument over "burden sharing" -- whether the allies are pulling their weight.

They are talking about the readiness of the Alliance to perform its prime function of deterring Soviet attack -- that is, deterring a conventional attack without having to threaten implicitly to go nuclear at a relatively early stage.

So last year's Nunn amendment is on hold. There is, however, a new Nunn amendment, addressing an alarming disparity in how much NATO and the Warsaw Pact are getting for what they spend.

The Europeans have been meeting the NATO goal of a 3 percent annual increase in defense outlays; the United States has exceeded it in the last four years by $120 billion. Even before these increases, however, NATO was spending more than the Pact countries, and producing less. Even in 1984, it was outproduced by ratios of 2 to 1 to 5 to 1 in major systems: 3,650 to 1,760 in tanks, for instance. The conspicuous exception was, as you would expect of an Atlantic alliance, in big surface ships.

The trouble, Sen. Nunn says, is that NATO countries wastefully develop and build competing systems. His new amendment fences off $200 million for development projects conducted with allies, requires the Pentagon to consider arms cooperation possibilities at an early procurement stage, and makes the Pentagon test competing U.S. and European systems against each other. It's nuts and bolts stuff, lacking in the drama of the earlier NATO initiative but pmising immense rewards for the common defense.