IN THE FALKLANDS, a war that should not

have happened is being pressed toward a very bloody end. Is the stage also being set for a longer- run misfortune? That is the alarming implication of some of the things Mrs. Thatcher said in interviews the other day, if we read them right.

The subject is Mrs. Thatcher's post-victory plans. These would seem to make it impossible for any Argentine government to accept her appeal to surrender before a final battle and to withdraw promptly. For her program denies Argentina the possibility of even asking for sovereignty in negotiations after the war.

What are those postwar plans? She will increase the British physical stake in the islands--"rebuild and rehabilitate and develop," the prime minister says. She will bring in new settlers: "I'm not talking about Argentinians." Security would have to be provided. Investment, settlers, security: it begins to sound not unlike Israel's policy on the West Bank. There are important differences, of course, but not all of them are comforting. Mrs. Thatcher now reveals, for instance, that the possibility of granting independence to the Falklands is very much on her mind.

The security factor requires another word. Britain, Mrs. Thatcher acknowledges, may have to defend the Falklands alone "for a number of years." (It would be interesting to know if she agrees with experts who suggest that Britain may need some thousands of troops on the islands to deter or defend against the various military moves a still-inflamed Argentina might contemplate.) But she hopes Britain will be able to form a "multinational force." With whom? The one country named is the United States: "You know, when the Americans asked us to join them in a multinational force in Sinai, I said yes, because it helped peace in that area. And I'm sure we'd have just exactly the same response from them."

Surely this is not on. What she proposes is not a "peace" force; the Sinai force separates the armies of two nations at peace. The United States would be joining an occupation force that would be resisted in one degree or another by Argentina and practically every other Latin nation.

We are not unmindful of the sacrifices Britain is making for a principle the United States fully and rightly supports. And it is also important to remember that the Argentines seized the islands and sought to hold them by force. It is necessary to ask, however, whether Mrs. Thatcher is plunging bravely but foolishly toward a military victory that Britain does not need and toward a postwar regime that Britain cannot possibly sustain. The alternative, a policy of military restraint and postwar modesty, is hard--and essential.