In diplomacy, frustration makes bad policy. A handbook demonstration of that unpleasant truth is the Reagan administration's frantic effort to respond to the Polish repression.

Sending food to Poland may strengthen an oppressive military regime; refusing food to a starving people provides the oppressors the excuse that short rations are caused by America's meanness. So far Western governments have resolved the dilemma in differing ways. Though some are donating food and supplies, our government has followed the opposite course. By rejecting official food aid and denying the Poles the protein derived from fishing in our waters, we run the risk of painting a pusillanimous grimace on the beloved visage of a magnanimous Uncle Sam.

If there are inherent contradictions in our sanctions against Poland there is an unavoidable ambiguity in our efforts to penalize the Soviet Union. By concealing its hand in forcing Poland's military takeover, the Kremlin has encouraged those yearning for East-West tranquility to dismiss the problem as an internal Polish affair.

Meanwhile we deceive ourselves when we fail to acknowledge the constricting limits of our power in a world divided into what amounts to classical spheres of influence. No U.S. unilateral action short of war can ever persuade the Soviets to permit Poland to violate the quintessential Communist principle of strict party control. Nor will we force the Polish government to soften its repressive measures so long as Soviet pressure persists.

We learned--or should have learned-- that hard lesson in 1956 and again in 1968, but faith in our omnipotence dies hard. So we turn to the last resort of the wistful--bold talk of economic reprisals that can have little but symbolic effect. Only the denial of wheat shipments (which constitute three-fourths of our Soviet-bound exports) could cause the Soviets serious discomfiture, but the Reagan administration ruled that out by yielding to the farm lobby, terminating the last embargo and promising not to repeat it.

Since we cannot by unilateral sanctions alter Soviet policy, our current shadow play exposes our weakness rather than strength. With our non-agricultural trade with the Soviet Union already whittled down by past sanction experiments, our punitive efforts appear more like those of a toothless poodle than a majestic lion.

Why, in view of all this, do we indulge in such posturing when our impotent gesture can bring no comfort to the Polish people? It is because, in trying to provide an outlet for our country's frustration, the administration is playing more for the effect in Peoria than on world opinion. If in addition some outraged citizens derive a moral glow from our flamboyant expression of disapproval, that is more therapy than foreign policy.

Stopping Aeroflot from landing in New York or even putting high-technology shipments on license are as much metaphors as lighting candles in our windows. One could not fault such symbolism if it did not encourage an unhealthy self- righteousness that contributes to the erosion of Western unity. Few at the top reaches of this administration seem aware that the policies of our European allies are conditioned by factual situations materially different from our own-- or if they know it they do not say so. Nor do they realize that the damage to Western cohesion may far exceed the exiguous effectiveness of sanctions that are more shadow than substance.

Though we can, at minimal cost, curtail our limited non-agricultural exports to the Soviet Union, it is like pushing thumbtacks into an elephant's hide. By contrast the denial of West Europe's trade would cause the Kremlin excruciating pain, but at a disastrous cost to several European countries.

Nor is the difference in opinion on the two sides of the Atlantic based solely on economic factors. Though American lives would not be immediately affected by deteriorating relations with Moscow, West Germany finds some residue of d,etente essential in alleviating the hardship of divided families in a divided country and securing the return of countrymen left behind when German territory was ceded to Poland.

If Americans understood why West European governments were more reluctant than ours to take punitive measures against Russia, they would show less sanctimonious pride in our bargain-basement gestures and feel less resentful at Europeans who do not automatically behave as we do. When will we ever learn that the maintenance of Western unity is far more important than hectoring Moscow ineffectively?