A West German with a long background in German- American relations passed through town the other day with a post mortem on West Germany's eventful month of May: Bitburg, Bergen-Belsen, the Bonn summit -- and the election in the state of North Rhine-Westphalia. The last was all but lost in the shuffle, but from what the visitor had to say, it is likely to have the most enduring repercussions on the Western Alliance and German- American relations.

It was a "crushing defeat" for Chancellor Helmut Kohl, the worst ever for his Christian Democratic Union party in West Germany's biggest state. It was also the third such setback in a row. And his own party is beginning to turn against him. It's not too early to measure the forces that will now be at work on Kohl to reverse the electoral tide. No sooner was the vote in than the chancellor let it be known that farmers, social-welfare beneficiaries and the unemployed would have to be given a higher priority -- at the expense of defense.

As the political imperatives are translated into government policies, this would almost certainly mean an inward turning away from matters the Reagan administration professes to care deeply about. One is "burden sharing" in NATO. The United States wants the Europeans to do more in the common defense. The administration is pushing for a new round of trade negotiations, with special emphasis on the European Community's agricultural subsidies. It speaks of a need for monetary reform (without, of course, conceding that the U.S. budget deficit and the U.S. dollar are a large part of the problem). On every count, Kohl's weakness works against these U.S. purposes. Kohl will be under heavier pressure to bow to public-opinion polls showing some 80 percent of the West German public in favor of a freeze on defense spending. But the United States will be in a poor position to cajole when the U.S. Congress is slashing away at defense-spending increases.

An outbreak of open trade warfare among the Western industrialized nations is never more than a tit-for-tat away. Protectionist fever is running high in Congress. Not to be outdone, the administration has rationalized the right to launch a new subsidy program for U.S. agricultural exports that flies in the face of principled opposition to the lavish subsidies offered by the European Community. The German visitor suggests the consequence of the U.S. subsidies may well be a European response in kind: "Any protectionism from the United States would get a strong reaction from West Germany."

The danger of a more generalized trade war must be seen in the light of what the Bonn summit failed to do. Seemingly as a result of French adamancy, the Seven could not even fix a date for a new round of trade talks. But appearances deceive. Kohl laid low at the summit, letting French President Francois Mitterrand be the spoiler. But Kohl "will be just as difficult as France" on the new trade talks, the West German visitor predicts.

Europeans never tire of telling us the Alliance can't operate when its heavyweight, the United States, is always changing governments -- and policies. The likely reverberations from North Rhine-Westphalia suggest discontinuity of policy is hardly an American phenomenon.