So much of our life, so much of our history, has been shaped by the bullet and the gun that it almost makes a mockery of humanity's persistent efforts to deal sensibly with the problems of this world. The assassination of Anwar Sadat is another cruel reminder of the heedlessness with which violence discards the dreams and plans of governments, leaders and average citizens.

That hard lesson was first borne home to me on Dealey Plaza in Dallas almost 18 years ago. The assassination of John F. Kennedy was the most fateful crime of the decade, not because he was more important or irreplaceable than the other victims who followed, but because so much more than a man was killed that day.

That was the day that a whole generation of Americans lost its innocence. Many of those people have been impelled, by forces more powerful than reason, to search for a cause commensurate in scale to the loss they felt. All of the bizarre conspiracy theories, including the one that led to last week's grisly disturbance of the remains of Lee Harvey Oswald, reflect an unsatisfied hunger for some way to rationalize the dreadful act.

Even now, they find it hard to accept that a man like Oswald could have ended the hopes that were embodied in John Kennedy. He had brought a new spirit and a new generation to American public life, and his assassination altered the political cycles of America in a fundamental way.

I have always felt that had Kennedy lived and won a second term against Barry Goldwater in 1964, as I think he would have, then neither party would have come back in 1968 with candidates representing a pre-Kennedy generation in American politics. I do not know who the nominees would have been, but I doubt very much that the choice would have lain solely among Lyndon Johnson, Hubert Humphrey and George Wallace on the Democratic side, or among Richard Nixon, Ronald Reagan and Nelson Rockefeller on the Republican.

With Kennedy leaving the White House at age 51, it is unlikely that either party would have willingly chosen a successor older chronologically and more antiquated politically than the retiring president.

America would have moved ahead with new leadership--and it might possibly have been spared the agonies of Vietnam and Watergate that cost us so much of our substance and spirit.

What Kennedy's loss meant to the whole nation, Martin Luther King Jr.'s murder in 1968 compounded--with special cost to the black community. It is 13 years since he was murdered in Memphis, and no one has emerged to replace him. There is no black spokesman who can command such an audience. And, more important, there is no one of any race who can evoke the moral indignation of the American people against the continuing policies which shame this nation and the world. The gap between a King and a Jesse Jackson or a Jerry Falwell is a very large one.

The assassination of Sen. Robert F. Kennedy, later in 1968, had other kinds of costs. He was a marvelously engaging man, always changing and growing. No one can know what kind of president he would have been, or even if he would have been nominated or elected in 1968.

What we do know is that his absence changed the character of the 1968 convention, and the subsequent history of the Democratic Party. I have always thought that if Kennedy had come to Chicago, somehow he and Humphrey would have become ticket-mates on a platform separating them from the Johnson policies in Vietnam.

Whether they would have won is problematical. But the convention would not have been poisoned by the cynicism of those who came without their slain candidate. The demand for restructuring the next convention would have been muted. Without those "reforms," neither George McGovern nor Jimmy Carter would likely have been nominated.

To bring this sad tale down to our own time, with the assaults on President Reagan, Pope John Paul II and President Sadat, the calamities--real and averted--are all too plain.

Reagan is the authentic voice of Reaganism. Had he not been spared, it seems very doubtful that the economic program he espoused would have made it through Congress--for good or ill. More important, the hope he has inspired in millions of Americans, who trust him more than they support his program, would have vanished.

Had Pope John been felled, not only would the Roman Catholic Church have lost its leader, the Solidarity movement in Poland--probably the most important new force on the face of the earth--would have lost its spiritual mentor and protector. And around the world, the symbol of strength in an ancient institution would have been mourned.

Of all these targets, Sadat was probably the largest historical figure, a man who was unique in personality, courage and vision. We sense already --and will, I fear, learn more--how irreplaceable he was, yet another lesson in the terrible tyranny of gun and bullet.