The struggle that opened with President Reagan's plea for his 1986 defense budget increases will be the ultimate test of his thematic presidency.

Reagan's magic touch derives from his talent for governing by themes -- hewing to broad and heroic lines, keeping our eyes on the larger contours of the forest and never being fussy about trees. This has won him great success, but now leaves him exposed because his themes often lack an essential spine of fact.

Go back for a moment to 1980. Reagan, with invaluable assists from the Iranian and Afghanistan crises, managed to peddle the dubious idea that Jimmy Carter (and even Gerald Ford) had been slackers in guarding the nation's military interests. It was the biggest fable of its sort since John F. Kennedy sold us on the "missile gap" 20 years earlier, and not less effective because Reagan actually seemed to believe it.

Indeed, the United States had idled along for a time while the Soviet Union leaped forward (not necessarily ahead) in missile development. But the lapse wasn't rooted, as Reagan repeatedly insists, in some fatuous complacency about Soviet intentions.

The lean years of strategic time- marking for the United States (1964- 73, approximately) were rooted in our dissipation of military strength in Vietnam. An under-budgeted strategic arm lost out to a money-eating regional conflict of scarcely third-rate importance. This was the basic fact, and the aberration was compounded by an apparent coincidence: The Soviets, stung by Khrushchev's humiliation in the 1962 missile crisis, were racing all the while to develop a huge ICBM inventory.

Once the disparity was recognized and assessed, at least two administrations, Ford's and Carter's, moved at a measured pace to correct it -- modernizing the warheads on the 1969-vintage Minuteman missiles; planning and laying down Trident nuclear submarines; and projecting new technologies such as the radar-evasive Stealth bombers. In short, strategic updating didn't commence with the advent of Reagan.

What did happen after 1981 was that the Reagan administration, while sponsoring a tax cut of some $750 billion, pushed to buy and build every strategic system in the catalog: B-1 bombers, more Tridents, new nuclear aircraft carriers, the MX super-missile, and now the very costly Strategic Defense Initiative. The lot.

Choice went out of style. Because the miraculous supply-side multiplier would make up for lost taxes, there was nothing the Pentagon couldn't afford. Severe choices were forced on many domestic programs in the name of economy, but the defense department stopped choosing.

This binge of buying everything in sight, coupled with the president's obstinate resistance to a tax increase, has brought on the presenbudget crisis. Not until its origins are clarified -- and they won't be by speeches like the one Reagan gave Feb. 26 -- can anything useful be done about it.

The president is right in saying that the drastic and indiscriminate defense cuts mandated by Gramm-Rudman (the mirror image of his own drastic and indiscriminate defense spending) are unsafe and costly. As much as a sense of priorities, defense needs stable budgeting on a cycle of realistic length -- five years, say, rather than yearly chops and changes.

But since it doesn't fit the fable, the president denies Congress the obvious remedy: new revenues. And he backs the refusal with an astonishing claim. "You, after all," he said, addressing us, the public, "have paid the bill for all we've accomplished these past five years."

We have? Now that is the most extravagant claim of all. The pivotal fact is that we haven't paid. We have borrowed about $1 trillion -- a third or more of it for the Reagan defense buildup. We have lived the heroic military life on plastic.

The Reagan defense-budget fable was pleasant while it lasted, and in certain limited ways stirring and right. But fables, in the end, are no match for the bookkeepers. The men in green eyeshades are about to have their inning.