THE ARRIVALL of West German Chancellor Helmut Schmidt in Washington this week sharpens the principal unanswered question of President Reagan's foreign policy. It is the Soviet question: how can the United States at once maintain a global balance of power and retain the full confidence of its allies? Some of the steps and stances the administration has taken to serve the first requirement have worked against the second. The policy of hanging tough on SALT and preparing an arms buildup before contemplating arms control, for instance, has nourished Western Europe 's considerable anxieties about the fading of detente and aggravated the leadership's difficulties in strengthening defense.

Nowhere is the tension more acute than in West Germany, where Mr. Schmidt has troubles of his own. Economic slippage and a faltering in the polls belie his election victory last fall. In May alone, he has met the repudiation of his defense policy by a regional party congress, his party's first fall from power in Berlin in three decades and the loss (in elections) of his principal European partner, outgoing French President Valery Giscard d'Estaing. There is also a fatigue factor: Mr. Schmidt faces the equivalent of an American president's third term.

The agitation of his party's left wing is crucial. The Germans, in NATO, decided 18 months ago to start deploying new American medium-range missiles in 1983 and, meanwhile, to negotiate with the Soviet Union to reduce the number of such missiles on both sides. But the Reagan administration has hung back from opening arms talks with Moscow, and the German left has seized on this to attempt to undo the whole deploy-and-negotiate decision. Over the weekend, Chancellor Schmidt seized the nettle. He challenged a declaration by a parliamentarian of his party that "the main danger to peace comes from the policy of the U.S. government." He threatened to resign if his party's left did not stop trying to undermine the new deployment.

The Reagan administration needs to meet Mr. Schmidt halfway. It helps that the secretary of state keeps alive the option of missile talks with Moscow but, given the doubts about Mr. Haig's authority that linger in Europe, the president needs to harden his personal commitment to this aspect of his policy. The administration, moreover, has to remove from its collective voice the recurrent and not-so-behind-the-hand suggestion that somehow Chancellor Schmidt is not a four-square Alliance man. It is the right moment -- Mr. Schmidt has just stood up gutsily to his left. Standing with him is the best way for President Reagan to serve his own ends.