It's a bit much, really: the ham-handed, tinhorn Argentine dictator challenging "this seat of Mars . . . this Realm, this England," in the name of some threadbare, 150-year- old territorial claim. The cheering, tearful dockside send-off for a 20th century British armada. The foreign minister resigning on a point of honor. And the embattled prime minister facing down a characteristically undecorous House of Commons--while evoking the glory of the Boer War: "Do you remember what Queen Victoria said: Failure?--the possibilities do not exist."

Good theater, yes. Comic opera, no. Margaret Thatcher means it. Sacred honor aside, the life of her government is on the line. And so, quite possibly, is the life of the military government of Argentina's president, Leopoldo Galtieri.

This is no old-fashioned exercise in bully- boy, gunboat diplomacy. Such are the passions, the pride and the political imperatives at work on both sides, that this is much more in the nature of a middling-power Cuban missile crisis. The Argentine invaders are in place and digging in. They can only be dislodged by force or by what might be called a combination of shuttle and slow- boat diplomacy.

That's the one bright spot: Secretary of State Haig can move faster, further in the jet age than the British fleet can churn southward through the Atlantic. The dark side is that you are dealing with a dispute that has defied reasonable compromise for 150 years; with an Argentine regime desperately in need of a triumphant distraction from its own economic malpractices; and, above all, with principles the British rightly believe the United States has a profound obligation to uphold.

If Pentagon spokesman Henry Catto was talking in military terms when he said the other day that "we have no idea of doing anything but walking right down the middle," that's fair enough. But if he was speaking to the merits, then what we are witnessing are the first sour fruits of the Reagan Doctrine on how to relate with "authoritarian," as distinct from "totalitarian," governments.

That's the doctrine that first caught the eye of Ronald Reagan when it was set forth in a 1979 Commentary article ("Dictatorships and Double Standards") by Jeane Kirkpatrick, now his ambassador to the United Nations. Briefly, it holds that authoritarian, anti-communist, military regimes of the right are generally less repressive on matters of human rights than totalitarian, communist states, and that Jimmy Carter was so stuffy about this that he needlessly alienated potentially useful allies.

The government of Argentina was specifically listed among those where "there is a far greater likelihood of progressive liberalization and democratization," than in, say, Cuba.

But in the meantime, by military coup, a quite sufficiently repressive military junta in Argentina has given way to an even harder-line crowd. The economy, with triple- digit inflation, is a shambles. There is growing public protest. Civilian rule by popular elections seems more remote than ever.

So the Falkland Islands grab has been a nice change of subject, joyously received.

The Reagan administration is caught between a rock (its burgeoning partnership with Argentina in the struggle against communist inroads in Central America) and a very hard place (the long U.S. special relationship with Britain, rooted in shared principles of self-determination and cemented by Atlantic Alliance membership).

The islanders once rejected a "Hong Kong" formula, which would have ceded sovereignty of their homeland to Argentina while leasing back administrative authority to the British and providing for a split of the returns from the oil that's supposed to be in the vicinity. Now under Argentina's occupation, they might have second thoughts. There lies the ultimate objective of preventive diplomacy