WHEN THE Falklands crisis began, people in many places warned each other not to take it as a Gilbert and Sullivan opera, but in fact many people--though not the British military--did take it as a Gilbert and Sullivan opera. They can do so no more. There were casualties earlier, but they occurred in a limited context, and negotiations promising the end of the conflict were still going on. Now casualties are being inflicted and taken in a much more open-ended context, and no negotiating process is alive. Real lives are being lost. The fate of governments, perhaps even the future of nations, is in the balance. There are ominous if uncertain global implications. It is war.

Mrs. Thatcher faced an excruciating dilemma: whether to stay in negotiations and risk losing a diminishing military opportunity, or to break off negotiations and accept the heavy risks and costs of battle. Leading as she does a democratic society, she is sure to be faulted as well as praised for the decision she took. But the important fact now is that she took it, and she took it in the name of a principle--to prevent conquest by force--that is central to Western tradition and to the requirements of world order. It is essential and it is right that the United States should be on the British side.

As difficult as was Mrs. Thatcher's decision to escalate military action, she will inevitably face a second and, in its way, no less difficult decision: the conditions on which to stop the fighting. She has removed from the table the concessions Britain had made in weeks of negotiating. That puts upon her an urgent requirement to formulate her terms soon. This will be no easy thing, especially if the military action produces, as it could, casualties and conspicuous losses of hardware but also something less than a prompt, clear-cut verdict. The claims of honor can generate conflicting judgments of how far and long a nation should fight on. Mrs. Thatcher's judgment will be of consuming interest not only to her people but also to her allies.

The curse of this war--of almost all wars--is that nations do not look ahead. The Argentine junta, for instance, actually seems to have believed it could pull off its little aggression scot-free. What is for the United States still mostly a diplomatic embarrassment, and for Britain still mostly a crisis of one government's policy, is for Argentina a climactic moment in its national life, an event likely to shape its politics and cultural orientation for years. The Argentines cannot be confident of what may come next.

At the United Nations, the secretary-general has passed the burden of diplomacy back to the Security Council, where Britain's veto guards any unacceptable turn. Yet a negotiated settlement remains essential. Blood thickens emotion. But if emotion can be kept from entirely blocking a rational consideration of national interest, then the differences between Argentina and Britain--differences which appear to be eminently susceptible to resolution--can surely be closed.