Margaret Thatcher was sore beset as it was: cries of "shame" from the opposition benches; the sacrificial sacking of her brilliant foreign minister, Lord Carrington; the British fleet, the queen's son aboard, steaming out of Portsmouth to a rendezvous that could be another Trafalgar.

And then Ronald Reagan spoke up at his first informal, spontaneous, unrehearsed Oval Office press conference. "We are friends of both sides," he said when asked about the Argentine aggression against the Falkland Islands.

"Friends of both sides" indeed. The words must have scalded Thatcher, who stood with us during the hostage crisis, who has defended Reagan from the sniping of other NATO allies, who has supported him, almost alone, on his economic policies and his nuclear strategy.

For other Americans, in this completely comprehensible, clear-cut, old-fashioned confrontation, the choice between our oldest ally and what one incensed member of Parliament called "a fascist, tin-horn junta" that has engaged in flagrant aggression, is a cinch. But not for Ronald Reagan. Twice he said, "We are friends with both sides." So much for gratitude.

To be sure, the British blundered badly. Obviously, their intelligence knew less about the imminent invasion than two London papers, the Sunday Times and the Daily Mail, both of whom had correspondents on the spot, awaiting the Argentine armada.

As the astonishing drama unfolded, it cried out for the pen of Evelyn Waugh. The Argentine marines swooped down on the island's 1,800 true-blue British inhabitants and their 600,000 sheep. Assurances that British citizens would receive the blessings of the Argentine Constitution added to the grisly hilarity. The Argentine Constitution has been suspended due to "a state of emergency," the mercies of which have been detailed in such books as Jacobo Timerman's "Cell Without a Number, Prisoner Without a Name."

The British fleet left port amid formal goodbyes reminiscent of World War I. In the two weeks needed for the 8,000-mile journey to the Falklands, while Britain prays that what is left of the British Raj can express British rage, statesmanship is needed. But the affair is anachronistic--it has no Soviet context, and Reagan seems to be at a loss.

He supported the U.N. resolution of condemnation but, from his later comments, his heart is not in it.

Argentine aggression was not in Reagan's scenario. He is off to Barbados to rally support for his Caribbean Basin initiative. His dubious grand design of building an anti-communist bastion in Latin America is largely dependent on close ties with the thuggish regimes in Chile and Argentina.

He got Congress to approve lifting the ban on military aid to those two countries, although not without restrictions. The Justice Department is standing firm on Chile--it will not certify progress in the bombing murder of Orlando Letelier. But Argentina, until it sent in the marines to subdue the shepherds, seemed sure of the official seal on human rights progress.

Argentina's lunge at conquest was initially a success. Last week, in the Plaza of May in Buenos Aires, the hundreds of mothers of "disappeareds" who march there weekly were joined by thousands protesting the murder of a young, left-wing pregnant woman. This week, after the subjugation of the Falklands, there were jubilant flag wavers, saluting a national triumph.

Asked if he is taking another look at military aid to Argentina, Reagan said, "We haven't considered it yet."

Thatcher, and the rest of the world, know what instruction Reagan has received about the limits of friendship with dictators. On the day of the Argentine invasion, he spent 50 minutes on the phone with President Leopoldo Galtieri trying to talk him out of it. So much for "quiet diplomacy."

But there has to be another reason why Reagan is so slow to anger. It may be that he is planning to use Argentines in another adventure in Latin America. They are, according to several reports, to be the backbone of a guerrilla force being mustered under our auspices to destabilize what we regard as an unacceptably left-wing regime in Nicaragua. Reagan can hardly denounce Argentines for overt aggression while counting on them for the covert kind.

The rest of his Carribean strategy is equally murky and unpromising. Much gush has attended the election turnout in El Salvador, without regard to the fact that the wrong side won.

Roberto D'Aubuisson, the right-wing bully who during the campaign ran a machete through a watermelon to show what he would do to the Christian Democrats and vowed to "exterminate" the rebels, is the big winner. Suddenly, he talks about land reform and pluralism in a manner as convincing as the constitutional guarantees being offered the Falkland Islanders.

Reagan can't choose between Britain and Argentina. He really doesn't know who his friends are.