WHILE SECRETARY of Defense Caspar W. Weinberger '38 prepared to visit American naval forces in the Persian Gulf last week, word arrived that an Army helicopter had sunk an Iranian ship spotted sowing mines in the Gulf. The incident proved what everyone has known all along: that the Iranians are responsible for the mines in the Gulf.

This minor military victory notwithstanding, America's Gulf force has not established its fitness to escort the 11 re-flagged Kuwaiti tankers through the waterway. The cruisers and aircraft carriers Washington has sent were designed for a major war with the Soviets not for guerrilla encounters with Revolutionary Guards in speedboats. The billion-dollar, 500-yard-long ships are fat targets in the narrow Gulf, where WWI-vintage mines have taken a disturbing toll.

UNDENIABLY, the U.S. military presence in the Persian Gulf is a military mistake. Yet it is also an overpowering political necessity, a much-needed opportunity for the US to restore credibility lost in the Iran-contra affair. And so far America has met that challenge.

However, the benefits will be lost if Congress invokes the War Powers Act this week. Should the Democratic leadership make good on its threat to push the measure through, allies in the Middle East will (justifiably) lose their emerging confidence in U.S. resolve. And the Administration's vehement opposition to the act signals the coming of a destructive constitutional clash with the legislative branch should the measure pass--which would be a time when such a confrontation could do U.S. foreign policy grave harm.

The executive-legislative spectacle is particularly ridiculous since a perfect way out of the quandary is available. In an address to the General Assembly, Soviet Foreign Minister Eduard A. Shevardnadze called for a United Nations peacekeeping force to insure the freedom of the seas in the Gulf. The State Department knee-jerk opposition to the plan is exactly what it appears to be: reactionary short-sightedness at its most blatant.

A United Nations naval presence in the Gulf would provide us with a way out of a difficult military situation. In addition, such a presence would clearly establish the world's support for U.S. action in the Gulf. At least this once, the Reagan Administration should use the United Nations as something more than a whipping-boy.