Will the real Margaret Thatcher stand up? The reason I ask is that so much has been made of the British prime minister's willingness to approve the use of U.S. fighter bombers based in Britain for the raid on Libya.

The Reagan administration would have us believe that there is at least one staunch, right- thinking ally in Europe. And this, it is said, is reason to hope that others will come to see the war against international terrorism our way.

Don't count on it, the more so if the administration's theory of the case proves wrong. The president's theory is that the raid "will not only diminish Col. Qaddafi's capacity to export terror, it will provide him with incentives and reasons to alter his criminal behavior. . . . This mission, violent though it was, can bring closer a safer and more secure world."

I don't think Thatcher believes that. I think the real Margaret Thatcher stood up on Jan. 10 in a news conference in London with American correspondents. What she said on that occasion nicely defines the fundamentally different way that Americans and most Europeans approach the problem of international terrorism. To dismiss this difference or to put it down to European "appeasement" is to invite crippling consequences for the Atlantic alliance if Ronald Reagan now finds himself regularly challenged to re- demonstrate his readiness to resort to military retaliation against terrorist targets.

The front-page headline in The New York Times read: "Thatcher Asserts Strikes on Libya Could Sow Chaos." Actually, she said "much greater chaos." And she went on to say: "I must warn you that I do not believe in retaliatory strikes that are against international law."

The context is important because it closely resembles circumstances leading to last week's raid on Libya. In December, terrorists had struck at Israeli airline check-in counters in the Rome and Vienna airports; among the dead were five Americans. Reagan had rejected a military reprisal, though he claimed then, as he did in the Berlin discoirrefutable evidence" of Qaddafi's complicity. But for the moment, he was only seeking British and European support for economic sanctions.

Thatcher would not even accommodate her good friend Ronald Reagan on economic sanctions, citing a long-held British view that they don't work. As for retaliatory strikes, in her Jan. 10 press conference she was even more adamant. "Please, may I remind you," she told the American correspondents, "that we have suffered over 2,000 deaths at the hands of terrorists, so we are well aware of the problems."

Conceding that the internal Irish Republican Army terrorism in Britain and Qaddafi's international terrorism are "wholly different," she nonetheu start to go across borders then I do not see an end to it. And I uphold international law very firmly." On Qaddafi, she was downright touchy. "Look," she said, "you do not have to tell me about Libyan terrorism. We saw it in our streets, the murder (in 1984) of a policewoman by shots fired from the Libyan Embassy."

So how is it that last week Thatcher was in the House of Commons vigorously defending the Libyan raid on the grounds of a "right of self-defense" of American citizens? Would not the same principle of international law apply to the Americans killed in Rome and Vienna? One can only surmise that because the issue was never pressed in January, it was easier to say no. This time Reagan sent a special envoy, U.N. Ambassador Vernon Walters, to plead his case. And he had another point: he had publicly warned of "further steps" if his allies denied him the concerted economic quarantine of Libya he was asking for, and if the American sanctions didn't do the job.

Whatever the case, the Margaret Thatcher of last January is a much better reflection of true British opinion, as evidenced in the House of Commons uproar last week -- and of European sentiments -- than was the Margaret Thatcher of recent days. Europeans have suffered far more heavily than Americans at the hands of terrorists, international as well as home-grown. They are far more experienced than we are in the use of overt and covert security measures and the pooling of intelligence.

Conceivably, a growing wave of terrorism might make them more receptive to U.S. urgings for tougher measures. But they are a lot less likely to move beyond their own methods when the urgings are accompanied by American suggestions that the Europeans are paralyzed by commercial greed or unawareness of the danger to their citizens as well as ours -- or simple cowardice. No more than Thatcher do they take kindly to lectures on terrorism from people who have not had to live with its horrors at home.