Thirty-nine years ago today, President John F. Kennedy was assassinated as he rode in a motorcade through Dallas. America felt itself changed forever.

The president was going to deliver a speech at the Trade Mart in Dallas, and to look at that undelivered address today is to look at a nation preoccupied with its security and the Communist threat - a time that at first appears so much simpler, but of course was not.

Today, we are preoccupied with other threats from other parts of the world, and President Kennedy's words of calm resolution are perhaps as compelling now as they would have been then.

Here are excerpts from that undelivered speech, which can be found in its entirety on the John F. Kennedy Library and Museum Web site: www.cs.umb. edu/jfklibrary/speeches.htm.

This nation's strength and security are not easily or cheaply obtained, nor are they quickly and simply explained. There are many kinds of strength, and no one kind will suffice.

If we are strong, our strength will speak for itself. If we are weak, words will be of no help. I realize that this nation often tends to identify turning points in world affairs with the major addresses which preceded them. But it was not the Monroe Doctrine that kept all Europe away from this hemisphere - it was the strength of the British fleet and the width of the Atlantic Ocean.

It was not General [George C.] Marshall's speech at Harvard which kept communism out of Western Europe - it was the strength and stability made possible by our military and economic assistance.

In this administration also it has been necessary at times to issue specific warnings - warnings that we could not stand by and watch the Communists conquer Laos by force, or intervene in the Congo, or swallow West Berlin, or maintain offensive missiles on Cuba.

But while our goals were at least temporarily obtained in these and other instances, our successful defense of freedom was due not to the words we used, but to the strength we stood ready to use on behalf of the principles we stand ready to defend.

This strength is composed of many different elements, ranging from the most massive deterrents to the most subtle influences. And all types of strength are needed - no one kind could do the job alone. ...

The strategic nuclear power of the United States has been so greatly modernized and expanded in the last 1,000 days, by the rapid production and deployment of the most modern missile systems, that any and all potential aggressors are clearly confronted now with the impossibility of strategic victory - and the certainty of total destruction - if by reckless attack they should ever force upon us the necessity of a strategic reply. ...

But the lessons of the last decade have taught us that freedom cannot be defended by strategic nuclear power alone. We have, therefore, in the last three years accelerated the development and deployment of tactical nuclear weapons, and increased by 60 percent the tactical nuclear forces deployed in Western Europe.

Nor can Europe or any other continent rely on nuclear forces alone, whether they are strategic or tactical. We have radically improved the readiness of our conventional forces. ... Finally, moving beyond the traditional roles of our military forces, we have achieved an increase of nearly 600 percent in our special forces - those forces that are prepared to work with our allies and friends against the guerrillas, saboteurs, insurgents and assassins who threaten freedom in a less direct but equally dangerous manner.

But American military might should not and need not stand alone against the ambitions of international communism. Our security and strength, in the last analysis, directly depend on the security and strength of others, and that is why our military and economic assistance plays such a key role in enabling those who live on the periphery of the Communist world to maintain their independence of choice.

Our assistance to these nations can be painful, risky and costly, as is true in Southeast Asia today. But we dare not weary of the task. For our assistance makes possible the stationing of 3 to 5 million allied troops along the Communist frontier at one-tenth the cost of maintaining a comparable number of American soldiers.

A successful Communist breakthrough in these areas, necessitating direct United States intervention, would cost us several times as much as our entire foreign aid program, and might cost us heavily in American lives as well.

About 70 percent of our military assistance goes to nine key countries located on or near the borders of the Communist bloc - nine countries confronted directly or indirectly with the threat of Communist aggression - Vietnam, Free China, Korea, India, Pakistan, Thailand, Greece, Turkey and Iran. ...

I have spoken of strength largely in terms of the deterrence and resistance of aggression and attack. But, in today's world, freedom can be lost without a shot being fired, by ballots as well as bullets. The success of our leadership is dependent upon respect for our mission in the world as well as our missiles - on a clearer recognition of the virtues of freedom as well as the evils of tyranny.

That is why our Information Agency has doubled the short-wave broadcasting power of the Voice of America and increased the number of broadcasting hours by 30 percent, increased Spanish language broadcasting to Cuba and Latin America from one to nine hours a day ... and taken a host of other steps to carry our message of truth and freedom to all the far corners of the Earth.

And that is also why we have regained the initiative in the exploration of outer space, making an annual effort greater than the combined total of all space activities undertaken during the '50s, launching more than 130 vehicles into Earth orbit, putting into actual operation valuable weather and communications satellites, and making it clear to all that the United States of America has no intention of finishing second in space.

This effort is expensive - but it pays its own way, for freedom and for America. For there is no longer any fear in the free world that a Communist lead in space will become a permanent assertion of supremacy and the basis of military superiority.

There is no longer any doubt about the strength and skill of American science, American industry, American education, and the American free enterprise system. ...

It should be clear by now that a nation can be no stronger abroad than she is at home. Only an America which practices what it preaches about equal rights and social justice will be respected by those whose choice affects our future.

Only an America which has fully educated its citizens is fully capable of tackling the complex problems and perceiving the hidden dangers of the world in which we live. And only an America which is growing and prospering economically can sustain the worldwide defenses of freedom, while demonstrating to all concerned the opportunities of our system and society.

It is clear, therefore, that we are strengthening our security as well as our economy by our recent record increases in national income and output - by surging ahead of most of Western Europe in the rate of business expansion and the margin of corporate profits, by maintaining a more stable level of prices than almost any of our overseas competitors. ...

For the first time in history, we have 70 million men and women at work. For the first time in history, average factory earnings have exceeded $100 a week. For the first time in history, corporation profits after taxes - which have risen 43 percent in less than 3 years - have an annual level of $27.4 billion.

My friends and fellow citizens: I cite these facts and figures to make it clear that America today is stronger than ever before. Our adversaries have not abandoned their ambitions, our dangers have not diminished, our vigilance cannot be relaxed.

But now we have the military, the scientific and the economic strength to do whatever must be done for the preservation and promotion of freedom.

That strength will never be used in pursuit of aggressive ambitions - it will always be used in pursuit of peace. It will never be used to promote provocations - it will always be used to promote the peaceful settlement of disputes. We in this country, in this generation, are - by destiny rather than choice - the watchmen on the walls of world freedom.

We ask, therefore, that we may be worthy of our power and responsibility, that we may exercise our strength with wisdom and restraint, and that we may achieve in our time and for all time the ancient vision of "peace on Earth, good will toward men."

That must always be our goal, and the righteousness of our cause must always underlie our strength. For as was written long ago: "Except the Lord keep the city, the watchman waketh but in vain."