IF SUFFICIENTLY mishandled, the farmers' trade war between the United States and the European Common Market can do real damage to both. A competition in subsidies would cost a great deal of money at a highly inconvenient time. Beyond that, unfortunately, trade quarrels have a way of generating a kind of bad temper that spills over into larger concerns like, for example, engineering an international economic recovery.

This time the United States is right. The Common Market is mismanaging its farm policy in a way that badly disrupts other countries' markets. The issue is once again export subsidies to get rid of those huge and embarrassing agricultural surpluses. If the Europeans were dealing with an unexpected and temporary fluctuation of output, American protests would be less vehement. But the impression here is that the Europeans regard their subsidized exports as an acceptable long-term solution to an imbalance that is getting steadily worse.

The international trade rules say that it's all right to expand your share of the wrold market. They also say that it's all right to subsidize your exports. But they go on to say that you mustn't do both. You are not permitted to use subsidies to take a larger share of the market, at the expense of other countries' exports. That's what the Common Market has been doing, but, as Sen. Mack Mattingly points out on the opposite page, the rules aren't being enforced. The United States, like every other government, subsidizes its farmers in many ways. But, over the past decade, it has not generally subsidized its exports. The recent sale of subsidized American wheat flour to Egypt was a deliberate warning to the Common Market of what may lie ahead if a sensible compromise can't be arranged.

In both Western Europe and the United States, the extraordinary technology of modern agriculture is producing more than farmers can sell. In this country, the Agriculture Department is again paying farmers to take land out of production. In Europe, the politics of overproduction is much more delicate. The Common Market is built on a series of understandings between France and West Germany; the larger market was supposed to benefit in particular German industry, and the compensation for it was to be high price supports for European--i.e., French-- agriculture. The thing has now gotten out of hand, but in the politics of the Common Market it's always easier to keep paying the farmers to produce, and then pay again to dispose of the stuff somewhere else. It's hard to believe that human imagination can't find a solution that serves even European interests less badly than a future of perpetually widening subsidy disputes.