So far, so good and so much for the Nervous Nellies in Congress, I was about to say. Then, WHAM, a mine blows a hole in the hull of the Bridgeton -- and also in my theory that the really risky part of this venture was not the escort mission but all the grand geopolitical baggage the Reagan administration tossed on board.

How was I to know until I saw a picture of the guided-missile destroyer USS Kidd tucked in snugly astern of the Bridgeton as it limped to port that the tanker's job was to protect the warship? As the captain of another U.S. vessel put it, the Bridgeton was ''acting as a deep-draft mine sweeper'' because ''it can take hits easier than we can.''

Some escort service! But never mind. The Bridgeton was deadheading to Kuwait to load up; no oil was lost; the hole can be patched, and so can my theory. It would have been nice if the first reflagged Kuwaiti tanker had made it through unscathed. But surely John Lehman's $1 trillion, high-tech, 600-ship (three-mine-sweeper) Navy can get the hang of it in time. It's only had since last March, when the deal was struck to reflag 11 Kuwaiti tankers, to get ready.

So let us suppose that the U.S. Navy accomplishes its mission in its narrowest sense: the safeguarding of Kuwait's oil lifeline. Iran's efforts to deal a crippling blow to one of Iraq's most valued allies will, accordingly, have been thwarted. And then what? Iran will presumably be under greater pressure to negotiate a cease-fire in its long and hideously costly war with Iraq. The principle of freedom of navigation will have been gloriously upheld. A modest Soviet presence in the strategic Persian Gulf will have been more than matched by an effective U.S. presence. And the balance of power in a vital region of the world will be firmly weighted in favor of the United States.

It sounds both sweeping and simple the way the administration lays it out. But the mine that blew that hole in the Bridgeton is a troubling reminder that the Reagan administration is an impulse buyer of foreign policies. The commitment to Kuwait was made without even preparing a careful, government-wide analysis of how things might go wrong. The current floundering around in search of mine-sweeping ''capability'' is evidence enough of the absence of contingency plans or of any sense of the importance of assessing risks in advance.

It's not as though the risks were not discernible both before and after the decision was made almost five months ago. Les Aspin, chairman of the House Armed Services Committee, set to work as soon as Congress was ''consulted,'' which was well after the administration had committed itself to gunboat diplomacy in the Gulf. In six speeches, he laid out some of the grimmer possibilities in persuasive detail -- with mines close to the top of his list.

His assessment covered other potential mishaps: attacks by high-speed patrol boats under the control not of the regular Iranian forces, but of the less reliable, fiercely anti-American, extremist Revolutionary Guard, for one example; accidental attacks such as the one by an Iraqi pilot on the USS Stark (an Iranian pilot in a fit of absent-mindedness once finished off an Iranian ship that had been hit by the Iraqis and was sitting at anchor waiting to be repaired); the use of Chinese Silkworm missiles by loose-cannon units of the Revolutionary Guard.

There is also the possibility that the Ayatollah Khomeini might decide to challenge the American presence head on, though most experts doubt it. What they don't doubt is that he will respond if the U.S. escort service works. Aspin dealt with these risks as well: third-country terrorist attacks against American installations; subversion or other forms of attack on other Gulf states friendly to Iraq.

Is the administration ready? Not if its response to the Bridgeton is any test. ''We have capabilities that are available when mines are discovered,'' said Secretary of Defense Caspar Weinberger, adding: ''What we don't do is talk about it.'' What they also don't do is have the ''capabilities'' available before the ''discovery'' is made -- the hard way. Asked why not, Weinberger said the Navy ''did not look'' for a mine in that stretch of the run up the Gulf ''because there have never been any mines in that area.''

Well, there had never been any yellow Mercedes trucks loaded with high explosives in the area around the U.S. Marine compound before one burst through the gate, crashed into the barracks and killed nearly 250 U.S. servicemen. There had, however, been terrorist acts against U.S. installations in Lebanon before the compound was hit, just as mines had damaged tankers in the Gulf before the Bridgeton was hit.

As with Lebanon, so it is in the Gulf in a larger sense: first came the commitment, for supposedly strictly limited purposes; then came the spinning out of grandiose strategic aims to justify the deployment of American forces in positions of high risk -- without weighing the risks. It is not too soon for the administration to begin trying to work out with Congress some degree of consensus on more realistic ends -- and more prudent means.