Secretary of State Alexander Haig suggests that Britain's successful landing on the Falklands--plus the sacrifices sustained by both sides--may make enough of a difference to permit negotiations to resume. Surely he is right. In its bridgehead and in its now-credible threat of sustaining operations ashore, Britain has something real to convert into political coin. Argentina may hope to raise the cost to the British and limit their military gains, but it cannot expect to boot them off the islands again. The 40 isolated marines it swept up on April 2 are one thing; the 5,000 troops ashore, with their fleet protection, are quite another.

Mr. Haig told CBS Sunday that what help Argentina is getting from its Latin friends did not yet amount to much and that he accepted at face value Argentina's word that it is not receiving Soviet assistance. It runs counter to just about everything known about the Argentine junta and Argentine pride, however, to assume that Buenos Aires would forgo available outside aid if it felt it were facing humiliation in its "Malvinas" campaign. Peru, for one, could help on one level, the Soviet Union on another. That could complicate the current determination of the American government, in backing the British, to avoid "another Suez": President Eisenhower undercut his British and French allies when they invaded Suez in 1956.

In that connection, we must ask, again: just what is Britain fighting for? Mrs. Thatcher has made clear the principle, or the collection of principles--democracy, self determination, resistance to aggression--but not the interest, nor the particular outcome, it wishes to see in the Falklands. Her foreign secretary, Francis Pym, could have clarified this matter yesterday, on ABC, but did not. The long-term arrangements to be made after British troops "repossess" the islands, he said, are "uncertain." There will be a British military presence in the short run and, in the long, "broader security arrangements." London will consult with the islanders and will talk with "many nations"--he did not cite Argentina--about "some pattern of defense." Mr. Pym apologized a bit for not being specific. "How can I be?" he asked.

In all due respect to an ally under duress, we must ask: how can he not be? Does the Thatcher government expect to retain the support it now enjoys for war aims it has yet to define? Does it believe there is any ultimate way to ensure the future of the islands without consulting the Argentines directly? It was, after all, inattention to the Argentine interest and the Argentine passion in the first place that got Mrs. Thatcher into the situation she is now attempting to redeem by arms. She has right and reason to oppose Argentine aggression but, now that she is fighting, she cannot ignore the central political reality: there is really only one other nation with which Britain must work out the fate of the islands --Argentina. The sooner that process can resume, the fewer men will die.