SINCE NO TURBINES are being loaded onto Russian freighters this week, the quarrel over the Siberian natural gas pipeline has shifted back from hardware to diplomacy. Europeans find it especially exasperating that the American attack on the pipeline keeps sliding from one issue to another. First, Americans objected that it would make Western Europe excessively dependent on Soviet energy. The Europeans thought they had answered that one when the White House told them the pipeline deal had to be abrogated to punish the Russians for martial law in Poland. The Europeans replied, and Americans said that the reply was irrelevant because, anyway, the pipeline would earn hard currency for the Soviet military budget.

Long ago, the Europeans began to suspect that the administration's real objection was not to the pipeline alone, but to trading with the Soviet bloc, in general, and particularly to the policy known as d,etente, to which that trade is crucial. West Germany's outgoing foreign minister, Hans-Dietrich Genscher, responds in the current Foreign Affairs. Despite his resignation Friday, his case deserves careful attention in this country, for his views are widely held in Germany.

He agrees that the Soviets' tendency toward aggression, notably in Afghanistan, is threatening. But, Mr. Genscher says, it's not a consequence of d,etente. He blames Soviet behavior on Western -- i.e., American -- failure to maintain the necessary balance of forces. Part of it was the American refusal to match the acceleration of Soviet military spending in the 1970s. Part of it was the refusal to respond to Soviet adventuring in, for example, Angola.

As for Poland, Mr. Genscher contends that Solidarity would never have taken shape in the absence of d,etente and the "intensive human and economic relations" that it provided between Poland and the West. There's a good deal to that. Europe's trade with the East is minor in economic terms; its real importance to the West is in the access that it requires communist governments to allow.

The administration wants to punish the Soviets for their part in the Polish events. The Germans think that if the pressure is great enough to be felt, it will only help the Soviet regime by cranking up nationalism. Instead, they counsel support for the military balance and pushing trade with its tendency to undermine communist authority. Those are differences of tactics that reasonable people can usefully discuss. In contrast, Mr. Reagan is now trying to use economic sanctions against the Western European democracies themselves to bludgeon them into a policy they consider wholly mistaken. That can only divide allies to the benefit of none but the Russians, Mr. Genscher warns, and there, unfortunately, he's right.