In reviewing the amazing proceedings of the Law of the Sea Conference that the Reagan administration scotched late last month, one thought continued to nudge forward in my mind, to wit: by common consent we are coming to the end of the postwar period. We are entering a new era. Its qualities are as yet unsettled, but the postwar period has had it.

The belief in the authority of the great powers that existed at the birth of the United Nations is no more. The norms and formalities of diplomacy that those nations established in the world are frayed or ignored. Of the great powers only the U.S.S.R. acts as though it had national interests, or as though it were a power of greater magnitude than, say, South Yemen. The rest participate in farce, the creation of a world order in which the United States will be the equal of Senegal, the world's fifth leading producer of ground-nuts--unshelled. Do the diplomats believe this? Over at our State Department, I believe, many of them do.

At the Law of the Sea Conference they were solemnly in the process of negotiating away still more American rights. Driven by the third world's assertion that the world's resources are "the common heritage of mankind," they were about to sign away the right to explore and to mine resources of the high seas without third world interference.

During the postwar period there was no problem in this domain. Deep seabed mining, though still in a fetal stage, was a high-sea right commensurate with catching tuna. But, as is characteristic of international diplomacy as we depart the postwar period, diplomacy, stupefied by the ideological pronunciamentos of a few backward nations, create problems where none had theretofore existed.

In this case a few ragamuffin nations, some snoozing in the last century, some snoozing nearer the Middle Ages, simply declared that the planet's resources are "the common heritage of mankind." The diplomats agreed. The backward nations insisted that seabed resources be forked over to a "Seabed Authority" from where they would be distributed to such worthies as populate the PLO. The diplomats agreed again.

Only a handful of men in the Reagan administration prevented us from going along with such an idiotic scheme. Now they are being branded as ideologues. Well, if they are ideologues their ideology is American national interest and a sense that the advanced nations know something that the backward nations do not know.

The ideology of those who refer to the world's resources as the "common heritage of mankind" is more easily codified; it is the ideology of Third World Socialism, and it is not esteemed for economic progress.

The Seabed Authority was, in economic terms, quackery. It would be an international organization modeled on the United Nations General Assembly, thus putting the backward nations of the world in control. It would have its own sources of revenue from licensing and taxation and would be able to strangle seabed mining. In other words, the Seabed Authority would have the same control over deep-seabed mining as the socialized Third World countries have over enterprise within their own impoverished boundaries. So much for the economic development of the seas.

The Law of the Sea was, then, the ultimate con perpetrated by the Third World ideologues of UniWorld, a scheme for pilfering the wealth of the civilized world while the backward nations sink ever deeper into the kind of barbarism that has recently been visited upon Uganda, the Central African Republic, Southeast Asia and other less notorious hellholes of the globe.

We are fortunate the United States has rid itself of this mischief. Had Jimmy Carter been in the White House you can be sure the United States would already have been bound over to the Seabed Authority, thus dooming the economic advances possible from American consortiums mining the seas.

What new adventures our diplomats will be getting us into in the years to come remains a mystery. But from the Law of the Sea Conference it is clear that many of them have simply given up on the notion of American national interest. They are apparently ignorant of how America came to be more prosperous than Senegal. They do not seem to believe that our advanced civilization has requirements or concerns that must be secure against grasping leaders in the Third World.

Only seven or so nations on earth possess the capacity and will to explore and mine the seas. They ought not to be shackled to the economic archaicism of the Third World.