Three years ago this week, Jimmy Carter became the first president in the post-Vietnam, post-Watergate era of fundamental changes facing America at home and abroad. He came to Washington, the only true political outsider of the century, hoping to rekindle, as he told the nation, "fresh faith in the old dream" -- and with a deeply held set of views about human rights and disarmament and morality in world affairs.He wanted to help shape, as he also said then, "a just and peaceful world that is truly humane."

Now, in the hard cold realities of 1980, this prisident would not hesitate to take any action -- including military -- to protect vital American interests in the face of a profound Soviet challenge. Now, in the light of the Soviet invasion of Afghanistan, this president could not support holding the Olympics in Moscow this year unless Russia withdraws its troops. Now, this president presides over -- and approves -- what certainly will be a constant buildup of U.S. military strength in the years ahead. And now, this president leads the nation into a period where the official emphasis on American goals has shifted to one of strength, resoluteness and of demonstrating the ability to counteract moves taken by potential adversaries.

All these represent more than a tempering of the views of Jimmy Carter. They reflect major changes in American attitudes about threats confronting the United States in the dangerous period ahead.

Before long, the president undoubtedly will express to the nation the long-range foreign policy and strategic implications that are now remaking American positions. A speech draft to this effect already is being prepared at the White House. The relationships between the United States and its traditional allies, as well as the new and inevitably closer ties with China, are certain to be among these long-term considerations. At the moment, of course, the principal focus remains on Soviet-American realtions.

Already the Soviet invasion appears to have had profound impact on world opinion. it's believed here that the Soviets did not anticipate the strong condemnation already expressed in many capitals around the world. Unless they move forward with complete disregard for the rest of the world and invade Pakistan, Iran and threaten Saudi Arabia, the view of the Carter administration is that they have made a strategic error.

What isn't known with any certainty by any American official is their ultimate goal. In other words, will Afghanistan be their ultimate targer or not?

For now, though, while the world pauses to see what Russia will do next, the initative is theirs.

President Carter and Soviet President Brenzhnev have exchanged messages via the "hot line." And the Russian leader has told Carter Soviet troops ultimately will be withdrawn.

There was nothing vague in Brezhnev's words to Carter. He flatly stated that as soon as the reasons which prompted the so-called Afghan "request" or "invitation" for Soviet troops to enter that nation's borders in force had disappread, then Russia fully intended to withdraw its military contingents from Afghanistan.

Whether this will prove to be true no one in Washington knows.

In the meantime, Carter has let it be known that the U.S. interests in the Middle East and Central Asia remain strong -- and that if the Soviet continue to maintain a warlike posture threatening Pakistan, Iran or other regions of the world, the United States is prepared to take whatever steps are necessary.

The United States will move expeditiously to provide Pakistan with improved defense capabilities, all the while working with a fairly large number of other countries in a kind of consortium arrangement. And the United States will pursue aggressively the opening up of military facilities for our Navy and Air Force personnel now deployed in the Arabian Sea and Indian Ocean area.

While no U.S. official wants to talk about permanent land bases because to strong objections from nations there to such a categorization, the prospects for establishing "service facilities" appear favorable in such places as Kenya, Oman, Somalia, or Egypt.

Whether the recent Soviet military moves and the American reaction to them result in a resumption of the worst of the Cold War tension, remains unanswerable. For nowthe ball is in the Russian court. Much depends on when -- and if -- their forces in fact are withdrawn.

The Moscow Olympics, for example, directly will be affected by the question. While the president prefers they be held, he would be opposed to their taking place in Moscow if

Soviet troops are still in Afghanistan. Alternatives exist. Another country could host the games, or several nations could stage individual competitions -- say boxing in Cuba or gymnastics in Japan.

Other ramifications to this radical departure of Soviet global policy already are influencing opinion in Washington and other capitals. Reactions in the Third World nations has been significant, and in some ways starting. At this point, for instance, Cuba has not spoken a word in support of the Soviet invasion -- something that would not have occurred in the last two decades.

At the same time, the attitudes of India as voiced at the United Nations has drawn expressions of comtempt and anger in Washington. When the Indian U.N. ambassador Friday night parroted what the Soviet Union, Czechoslovakia, and Vietnam had said, it led an official to liken that speech to one that could have been written by a Soviet puppet.

Jimmy Carter came to office believing that the concept of detente was in the best interests of everyone -- of Russians, of Americans, of all the people in the world. That view has not changed. Control of nuclear weapons, increasing trade and tourism, and gaining greater knowledge of each other remain optimum goals.

But obviously the naked Soviet action in Afghanistan has shaken him. Their invasion of a small, relatively defenseless, nonaligned, deeply religious contry represents virtually the most marked change in Soviet policy during his lifetime. And one that holds profound significance.

These closing days of 1979 and the advent of the '80s have brought a reassessment of many American policies and positions. Until the Afghan invasion, the president was preoccupied with Iran. Now that situation, too, has been affected by the Russian moves.

It's known in Washington that Iranian leaders -- including the Ayatollah Khomeini -- are deeply concerned about the Soviet invasion of neighboring Afghanistan.But whether this will have the effect of forcing them to resolve the hostage question cannot be determined.

The American-Iranian deadlock and the corresponding U.S. refusal to send them spare parts for their military equipment -- an action joined by all of the U.S.'s major trading partners -- has weakened Iran militarily. That point ad its vital connection with the Soviet march of troops so close to its border has been made directly to Khomeini by diplomats and others in recent days.

Other problems have plagued Iran, internally and externally. Such historical enemies as Irag and Russia pose threats to their sovereign territory. Inside their borders, dissension and armed conflict continue in different corners of the country.

In the face of these situatons, the Carter policy remains the same: to resolve the kidnaping of our citizens, not to yield on matters of American principle, but at the same time to permit Iran to save face and accomplish its purposes.

Already, there has been a substantial lowrering of Iranian demands. The president has no objection to their airing before the world their grievances against us, against the shah, and against the taking of Iranian assests out of the country. He refuses to suport any tribunal that would "try" the United States or the shah. But Carter's policy is to remove any impediment that might exist to keep Iran from seeking legal redress concerning their country's assets in U.S. or other courts.

Again, there's no assurance this will work. But in this as in other matters the president proceeds these days with what appears new confidence and assurance. He begins his fourth year in office facing certainly some of the most difficult if not perilous questions in many years. But he sees a nation more united and holds a perception of world more focused on the harsh new realities of late-20th-century life.