When former national security adviser Robert C. McFarlane returned to the witness stand last week, the sharp focus was on his challenges to the testimony in the Iran-contra hearings of his old friend and helpmate, Lt. Col. Oliver North. All but lost in the news accounts of his point-by-point rebuttals was his broader challenge to the performance of the administration.

It figured: lively contradiction is catchier than sound counsel. But McFarlane's insider's analysis of ''what went wrong,'' when Ronald Reagan and Congress first found themselves at daggers drawn over aid to the Nicaraguan contras deserves a lot more attention than it got. He was, after all, an influential participant for a time in this administration's policy making -- a part of the problem as congressional critics see it.

Yet, there he was propounding a solution to the age-old conflict between Congress and the executive branch over foreign-policy-making prerogatives that even the administration's severest critics would find hard to fault.

Perhaps more to the point, he was laying out lessons learned the hard way that a supposedly reformed Reagan administration still seems unable to grasp -- if its current handling of its policy to police the Persian Gulf with naval forces is any test. Consider, first, the rules McFarlane, with benefit of hindsight, would now apply:

''Any foreign-policy issue that involves threats to truly vital national interests, the resolution of which will take significant funding for a long period of time, had better have the understanding and support of the American people going in,'' McFarlane told the committees. ''That support is engendered by vigorous public debate.'' Instead, the president made his first mistake when he ''pinned his hopes in 1981 on a covert action'' to accomplish so large a mission.

Congress was by no means blameless; indeed, it ''worsened matters considerably'' by shutting off military aid when the ''secret war'' inevitably became public knowledge, McFarlane argued. But ''notwithstanding the congressional vacillation,'' he went on, ''once the Congress spoke, the administration should have listened, even though to do so would have surely involved a tragic loss of life among those in Nicaragua'' who had become identified with the contra cause.

Congressional opponents of the contra policy could hardly have asked for more. But there was more. The administration's decision to pursue ''private support'' was doomed to fall short of the needs. By ''going it alone,'' the president ''almost guaranteed'' the current political furor. It was ''simply wrong,'' on an issue so ''poorly understood by the people,'' for the president to behave as if he ''is not accountable to anyone.''

Now what does this have to do with the administration's decision to ''re-flag'' 11 Kuwaiti tankers and provide them safe escorts through the Persian Gulf? Just this: even as McFarlane was testifying, the administration's top policy makers were meeting downtown with congressional leaders, telling them that the administration has no intention of changing its mind in the face of opposition by what would appear to be clear majorities in both the House and the Senate.

True, the issue has been publicly -- and angrily -- debated, but to no practical effect. For the commitment to Kuwait was made and accepted well before Congress was even notified, let alone consulted in any genuine way. Secretary of State George Shultz recently added insult to the original injury by stating publicly that, ''At the time we couldn't even get members of Congress to listen as we tried to brief them.'' The administration's own, official chronology of its dealings with Kuwait and Congress in the first three months of this year shows that statement to be, quite simply, false.

After weeks of dickering, the United States was whipsawed by Kuwait (which was brandishing counteroffers from the Soviets) into a hurried offer on March 7 to reflag the Kuwaiti tankers. The offer was accepted March 10. The first suggestion of a briefing of relevant congressional committees was made to staff members by a State Department official on March 12. Not until the end of March did members of the Senate Foreign Relations Committee and the House Foreign Affairs Committee get anything in the way of a formal briefing -- in closed session.

Congress was neatly trapped. The issue it confronted was never whether ''reflagging'' was a good idea, but whether a vote to break a U.S. government commitment would be a bad idea.

So off we go again, on what is generally agreed to be a potentially high-risk, high-cost mission, with multiple purposes ''poorly understood by the people,'' and with the president behaving as if ''he is not accountable to anyone.'' It is sad to say, but only Bud McFarlane, among the close foreign-policy advisers of the Reagan presidency, seems to see the value of having the ''understanding and support of the American people'' on issues threatening our ''vital interests.'' And he's no longer on the job.