AMERICAN strategy in the Brussels trade talks is now do or die. Unless the European Common Market is prepared to make substantial changes in its highly subsidized and protected system of agriculture, the Americans say that they are prepared to walk out and let the whole negotiation collapse. Some of that may be posturing, but after a certain point the posture becomes the reality. Without a better offer from the Europeans than they have made so far, the American negotiators have little chance of getting an agreement through Congress.

This trade agreement is not a conventional treaty. It will -- if it succeeds -- require Congress to enact a large package of changes in American trade law. Congress isn't likely to do that unless there's pressure from American exporters who see large opportunities in the new trade rules. It's not only a matter of American farmers who want to sell more to Europe. Without European concessions on agriculture, the Third World isn't likely to see enough for it in the agreement to induce it to accept the new rules on patents and copyrights that American manufacturers, publishers and entertainers urgently want. European agriculture has become the keystone of the whole structure.

The talks in Brussels, which will run through this week, are supposed to be the culmination of four years of preparatory bargaining. But at least in the first two days, the deadlock showed no signs of breaking. That can change. As in a labor negotiation the stakes are high enough that no one can afford to give in except after a great demonstration of effort, probably at 3 o'clock in the morning.

It's also possible that this week's struggles could end with nothing decided. If that happened, it would still be possible to rescue the agreement, but it would be a race against the calendar. The real deadline, March 1, is set by the American law that sends the agreement through Congress under the fast-track process, protecting it from being nibbled to death in the committees. But to get the final texts completed by March won't be easy, even if there's a deal on the main points this week.

As the Brussels talks began, thousands of European farmers congregated there to mount a collective tantrum in which they expressed their feelings by throwing stones and tearing down traffic signs. But the rest of the world has a right to complain about the circumstances in which they are being supported. It's not only that Europe severely limits agricultural imports (as the United States, to its shame, also does in some crops like sugar). Worse, the European price supports are so high that they generate enormous surpluses, which the Common Market routinely dumps abroad -- that is, it subsidizes these sales as much as necessary to undercut other sellers. That's enormously destructive.

All the world, but especially Europe, North America and Japan, have benefited hugely from the expansion of world trade. To keep it expanding requires modernizing the basic rules. Failure to accomplish that because of a quarrel over farm subsidies would be a melancholy commentary on the quality of the political leadership of the world's richest countries.