Can Shultz do it?

In resigning, Secretary Haig defined the task of his successor. The ''lack of clarity, consistency, or steadiness of purpose'' has indeed made a shambles of the Reagan foreign policy. Overcoming that deficiency proved to be beyond Haig's capacity. It will clearly tax the impressive talents of Secretary Shultz.

Yet a coherent strategy is essential for an effective policy for coping with Soviet rivalry and an interdependent world. It must rest on a sound grasp of international realities and clarity regarding US objectives and priorities. Both requisites have been sadly lacking under Reagan and indeed earlier. On issue after issue, policy has been based on illusions about actual conditions or confusion about purposes or both. The consequences have been manifest in the military program, arms control, the Middle East, China-Taiwan, and, above all, relations with the Soviet Union and with allies.

Allied relations, for example, which are crucial for dealing with the Soviet Union as well as for constructive economic cooperation have seldom been in worse shape. The Europeans are not blameless, but the fault lies mainly with the administration. A look at the handling of the Siberian gas pipeline will illuminate why this is so.

First, the President's order barring European subsidiaries and even licensees from using General Electric technology for the pipeline compressors seemed utterly cavalier. The Reagan concerns - European dependence on Soviet gas and the $8 billion to $10 billion of hard currency revenues for the USSR - are not groundless. But despite these factors the Europeans had earlier decided to go ahead, relying on safeguards against Soviet leverage and eager for jobs for depressed industries and for future trade from the revenues.

Taken without warning, and after the Versailles summit, the unilateral US action infuriated all the European allies. It led Andre Fontaine of Le Monde to call the White House a small coterie of ''California Gaullists'' which ''takes little account of the opinions and even less still of the interests of others, even if they happen to be America's allies.''

Equally disturbing was the rationale. Officially it was the repression in Poland. Allied restraint on Polish credits may influence the Polish regime. But delaying the pipeline even a year or so would hardly modify Soviet policy on Poland.

Nor is the credibility of the administration enhanced by suggestions that the Soviet economy is verging on collapse. It has major problems but no serious Western expert would support that extreme view. Indeed the greatest Soviet import need arises from the enormous grain shortfall; yet Reagan ended the Carter grain embargo and is considering extending the export agreement, while opposing the pipeline. For the Europeans that is inconsistent and hypocritical.

Basically, the allies view East-West trade as reinforcing stability in Western Europe by giving the Soviets a stake in avoiding tensions there. In general, like Mr. Shultz, they think it a poor instrument for diplomacy. One thing is certain: trade embargoes cannot be effective unless the allies join in imposing them.

At bottom, the pipeline issue is a matter of priorities. The turbine embargo is hurting allied unity far more than the Soviet Union. Thus it seems willful and quixotic. It gravely exacerbates the effects of disputes over trade, security, and economic issues, which are dividing the alliance. The current London Economist details the steady alienation of President Mitterrand, who had gone to great lengths to improve French relations with the US and to take a strong anti-Soviet stance.

In microcosm the pipeline displays the confusion about reality, objectives, and priorities that skews so much of the Reagan policy.

Will Secretary Shultz be able to correct the situation - to achieve coherence where Haig failed? He seems well equipped by character, intelligence, and his approach to policy. His discussion of the Middle East at the confirmation hearing showed a solid grasp of the situation, of US interests, and of the priorities in dealing with it. His lack of experience in security, arms control, and related fields is an initial handicap, but it can be overcome by the effective use of expert advisers.

The harder question is whether the White House will permit him to pursue a more coherent course. The difficulty starts with the President and his most trusted advisers. Their knowledge of foreign affairs is rudimentary (to put it charitably) and their instincts and preconceptions are part of the problem. Their outlook is simplistic, reflecting a nostalgia for earlier US dominance and a less complex world.

Mr. Reagan might take a lead from Mr. Truman, who also came to the presidency with limited qualifications in foreign affairs. After a false start with Secretary Byrnes, he found a comfortable relation with Secretaries Marshall and Acheson. As Acheson put it, there was no doubt who was President, but Truman relied heavily on his secretary of state to navigate and conduct foreign policy.

Unless President Reagan gives Secretary Shultz that kind of trust and backing , he will not be able to bring sense and order into US policy. And then the outlook for the US and its allies will be perilous indeed.