AS I WRITE this, a third of a century has passed since I first entered Congress; five years have passed since I resigned the presidency.

When I resigned that office, I left unfinished the work that meant more to me than any I have ever been engaged in: the establishment of a new "structure of peace" that might prevent a major war, and at the same time maintain the security of the western world during the balance of this century. Since that time, the position of the United States relative to that of the Soviet Union has seriously worsened, and the peril to the West has greatly increased.

During most of my presidency, America was fighting a bitter war in Vietnam. During all of my presidency, we were engaged in a "war" with the Soviet Union. That struggle with the Soviets will continue to dominate world events for the rest of this century.

The Soviet Union today is the most powerfully armed expansionist nation the world has ever known, and its arms buildup continues at a pace nearly twice that of the United States. There is no mystery about Soviet intentions. The Kremlin leaders do not want war, but they do want the world. And they are rapidly moving into position to get what they want.

In the 1980s, America for the first time in modern history will confront two cold realities. The first of these is that if war were to come, we might lose. The second is that we might be defeated without war. The second prospect is more likely than the first, and almost as grim.

The danger facing the West during the balance of the century is less that a nuclear holocaust than it is of drifting into a situation in which we find ourselves confronted with a choice between suicide and surrender -- red or dead.

The next two decades represent a time of maximum crisis for America and for the West, during which the fate of the world for generations to come may well be determined.

Soviet ambitions present the United States with a strategic challenge of global proportions, which requires a strategic response. Piecemeal temporizing will not do. Angola, Ethiopia, Afghanistan, South Yemen, Mozambique, Laos, Cambodia and South Vietnam all have been brought under communist domination since 1974. Iran has been plunged into bloody chaos and turned overnight from a bastion of western strength to a cauldron of virulent anti-westernism, its oil treasures lying provocatively exposed to lustful Russian eyes.

Since World War II, the Soviet military buildup has been continuous and the Soviet expansionist pressure has been unrelenting. Moscow has fished assiduously in the troubled waters left in the wake of the dismantling of the old colonial empires. It has blockaded Berlin, fomented relations in Latin America, Asia and Africa, aided aggression in North Korea and North Vietnam. It has trained and subsidized guerrillas, disrupted elections, shot down unarmed planes, sponsored coups, shot refugees, imprisoned dissidents. It has threatened, blustered, connived, conspired, subverted, bribed, intimidated, terrorized, lied, cheated, stolen, tortured, spied, blackmailed, murdered -- all as a matter of calculated, deliberate national policy.

The basic rule of Soviet behavior has been the one laid down years ago by Lenin: Probe with bayonets. If you encounter steel, withdraw. If you encounter mush, continue.

The question is which will the Soviets encounter: steel or mush?

There are many today who suggest that American civilization is suffering a terminal illness; that we are witnessing the beginning of the end of the West. Some American opinion leaders view this with despair. Some, especially in darkest academia, see it as the logical and overdue result of our being on the wrong side.

What America does suffer from is not itself a terminal illness, but rather a sort of creeping paralysis that could become terminal unless treated. Together with our allies in the western world, we have the capacity to survive, to prosper, to turn back the challenges to our security that are being mounted with increasing force. The question is whether we will use that capacity.

The naive notion that we can preserve freedom by exuding good will is not only silly but dangerous. The more adherents it wins, the more it tempts the aggressor.

The situation today is ominously reminiscent of the period preceding World War II.

Revolution itself is neither inherently good nor evil. But what the United States confronts today is the advance of a tyranny marching under the banners of revolution: one that seeks to replce democracy with despotism, all spuriously in the name of "the people."

For many years, Afghanistan -- remote, landlocked, a harsh mountain region of primitive tribesmen as rugged as the land they lived on -- was treated as a metaphor for all those distant events that glazed the eyes of the American reader.

But in real life, Afghanistan is much more than that. With Iran on the west, Pakistan on the south, China on the east and a 1,000-mile border with the Soviet Union on its north, Afghanistan has traditionally been one of those points where the great thrusts of empire met.

Today, Afghanistan is a testing ground for an ominous new phase in the Soviet expansionist drive.

A bloody Soviet-backed military coup in April 1978 suddenly ousted President Mohammed Daoud, who was promptly murdered, and installed in his place a stridently anti-western, Marxist regime.

Soon nearly every government ministry, as well as the 100,000-man Afghan army, had Soviet "advisers."

This abrupt renewal of centuries of Russian pressure against its repeatedly extended Asian borders sent shock waves through Afghanistan's already weakened immediate neighbors, Pakistan and Iran.

Less than 10 months later, in fact, the shah's regime had fallen in Iran and leftist guerrillas staged their first takeover of the U.S. embassy in Tehran on the same day that the U.S. ambassador to Afghanistan was dragged from his car and murdered.

However, fiercely independent Moslem tribesmen launced a jihad, or holy war, for control of their country and of their lives. Insurgents sold their cattle and their wives' jewelry to buy ammunition. The rebels fought Soviet-made tanks by starting landslides. They rushed directly into the machinegun fire of the tanks and overwhelmed them, armed with nothing more than sticks and iron bars.

The government's army suffered from purges, desertions and defections to the rebels. By late 1979 it was doubtful that the communists could have survived another rebel spring offensive.

In a September 1979 coup, Prime Minister Noor Mohammed Taraki was ousted and executed by his No. 2 man, Hafizullah Amin. But Amin made little headway in putting down the rebellion. In a carefully prepared and brazenly executed move, the Soviets invaded Afghanistan on Christmas Eve. Amin and his family were killed; a reliably pliant Soviet puppet, Babrak Karmal, whom the Russians had kept hidden away in reserve in Eastern Europe, was put in as Amin's replacement.

Russia came one county closer to achieving its goals -- now within tantalizingly short reach -- of a warm-water port on the Arabian Sea and control over the oil of the Persian Gulf.

It did not occur in isolation. It was part of a pattern. And that pattern is what presents the challenge.

Communist regimes have taken power not only in Eastern Europe, but also in China, North Korea, all of Vietnam, Cambodia, Laos, Afghanistan, Ethiopia, South Yemen, Angola, Mozambique and Cuba. So far, no country that has come completely under communist control has escaped from that control. tTwenty-one countries are now in the communist orbit. The communist powers are advancing all over the world, and the West is retreating.

At the time of the Cuban missile crisis in 1962, the United States had overwhelming nuclear superiority, in the range of 15 to 1 or even more.

But since 1973 the Soviet Union has been spending three times as much as the United States on strategic nuclear arms.

Unless the United States drastically increases its military budget, the Soviet Union by 1985 will have unquestioned nuclear superiority, overwhelming superiority on the ground, and at least equality at sea. In sum, unless we act fast and now, the period of the mid-1980s will be one of maximum peril for the United States and the West.

The most dramatic of Russia's vulnerabilities lies in the deep and perhaps irreconcilable differences between the Soviet Union and China. In the long run, china may pose an expansionist threat to the West. But for the present, China fears the Soviet Union and needs the West.

A second vulnerability stems from the nature of the communist system. No people has ever freely chosen to live under communism. No nation remains under communist rule except through force.

A third vulnerability, one that is potentially a decisive western advantage, lies in the fact that economically, capitalism works and communism does not.

As we survey the world's economies, we find that the United States, Western Europe and Japan together have a gross national product four times as great as that of the entire Soviet bloc.

The communist nations have the advantage that being totalitarian, they can allocate their resources as their leaders choose, to serve the ambitions of the rulers rather than the needs of the people. But if there is to be an arms race, and if the West decides to compete, the West has the economic power to win it. The Soviets know this.

The capitalist system works on the basis of the profit motive economically. The Soviet system works on the basis of the profit motive militarily and territorially. When the Kremlim calculates that it has more to gain than to lose by an act of aggression, subversion or intimidation, then it will engage in such action.

The United States had time to recover from a naval Pearl Harbor, and it had ample warning of impending war. We could have less than 30 minutes' warning of a nuclear Pearl Harbor, from which we would have no time to recover.

There is no time to lose.