THE SIEGE; The Saga of Israel and Zionism. By Conor Cruise O'Brien. Simon and Schuster. 798 pp. $24.95.

WHY WOULD SOMEONE named O'Brien write a saga of Israel and Zionism? Good question, the Irish diplomat, journalist and historian observes in his prologue to the book. The answer, he says, lies in his past -- in his identity as a member of a nationality, Irish, that has experienced stigmatization, and in his identity as the son of a lapsed Catholic growing up in a southern Irish sea of disapproving believers. These identities, he says, helped him form a bond with the story of a people whose stigmatization has been profound and whose experience with disapproval has been catastrophic.

As a result of that bond, he discovered in himself a compelling interest in the history of the Jews and in their efforts to create, or revive, their own national home. The story of that effort, he believes, is "inherently astounding" -- indeed, "the greatest story of modern times." His aim, he says, is to tell that story for the general reader, and to show not only how Israel came to be what and where it is, but, perhaps more important, "why it cannot be other than what it is."

The "why it cannot be other that what it is" -- a formulation that seems curious when it is first encountered in the book but that ultimately becomes central to O'Brien's withering conclusion -- stems directly, he believes, from the national, religious and historical forces that gave life, shape and urgency to the modern Zionist endeavor.

Zionism did not begin with Theodor Herzl. It was the dominant theme of the Jewish exiles 2,500 years ago, as they wept by the waters of Babylon for their lost home. And it was the dominant national theme during the two millennia following the last exile, which began after Rome subdued the Jewish revolts and changed the name of Judea to Palestine. After that, wherever they found themselves in the diaspora, Jews prayed, three times a day, for the Return.

Through the centuries, this religious Zionism propelled Jews, individually, to risk the journey to their ancestral home, some of them to settle in it, even though it was in the hands of a succession of conquerors hostile to their presence. For the vast majority of Jews, however, the Return was reserved for the Messianic future.

By the late 19th century, however, some Jews, mostly secular ones, decided they could wait no longer. For them, the Return was a matter of this world, and of increasing urgency. In Russia, where 5 million Jews lived, anti-Semitism became official policy, and murderous pogroms commonplace. And in Western Europe, traditional Christian anti-Semitism, though attenuated by the Enlightenment, was joined by a new and more nationalist, racist kind. Conversion could no longer save the Jews. For these new anti-Semites, the Jews were perpetually alien -- not French or German but a race apart; and for at least some of them, such as Eugen Duehring, the Jewish question was to be solved by "killing and extirpating."

Theodor Herzl read those words at the age of 22. "The effect of Duehring's book upon me," he later wrote, "was as if I had suddenly been hit over the head."

Herzl, who with his tall stature and long, black beard looked like an Assyrian king, was an assimilated Viennese Jewish journalist who, having been hit over the head, saw that his people faced an ultimate kind of danger; and, in 1896, he launched a program of political Zionism that, 52 tumultuous years later, led to the creation of a Jewish state.

O'Brien tells the story not only of those years of Zionist struggle but also of the Israel that that struggle produced. He describes the country's wars, alliances, leaders and policies; its daunting dilemmas, both strategic and moral; the recurrent patterns of lost opportunities and self-defeating extremism that continue to characterize the Arab-Israeli conflict; and the profound ways in which the Holocaust has seared the vision, soul and imagination of Israelis, as well as Jews elsewhere, deeply influencing the ways in which they see, feel and trust.

BUT OF ALL the aspects of this rich, well-crafted, sometimes disturbing and often illuminating book, surely the greatest attention must be paid to its stark conclusions, written last summer. Focusing on the diplomatic maneuvers involving Israel, Jordan's King Hussein and the Palestine Liberation Organization, O'Brien contends that Israel cannot possibly take "bold steps" for peace. "Bold steps," in this case, would mean giving up all of the West Bank as well as sovereignty over Jerusalem -- the minimum that, O'Brien says, Hussein would be willing to accept in order to risk signing a peace treaty. But no conceivable Israeli government, O'Brien believes, would ever contemplate giving up all of the West Bank -- and act that would, by itself, ignite unprecedented civil strife among the country's Jews; and the idea that Israel would ever give up Jerusalem -- Zion itself -- is, O'Brien is convinced, simply absurd. Moreover, O'Brien goes on, even if an Israeli government did do the politically suicidal thing, and offered Hussein what he would accept, the PLO would never go along with the deal; powerful elements within it, which are steadily increasing in strength, want all of Palestine and no Israel. With the support of Syria, these elements would brand any PLO leader who would participate in such a deal as a traitor, and would do everything possible to destroy both the leader and the deal. And even if a Jordanian-Palestinian confederation, or even a Palestinian state, were in fact set up, despite all of these impossibilities heaped on impossibilities, bloody civil war would immediately erupt, with attacks launched against Jordan and Israel as well as fellow Palestinians, and the entire enterprise would quickly collapse.

O'Brien concludes that "The idea of Israel withdrawing to its pre-June 1967 territory, and living there behind secure and recognized frontiers, in peace with all its neighbors, is an agreeable international pipe dream." Nor, he says, can he condemn Israel for not carrying out such a withdrawal. It simply can't, he insists. Israel was created to give a people hounded everywhere, and repeatedly and helplessly killed in ways and on a scale no other people has experienced, a secure haven; such a withdrawal would deeply compromise that security.

As O'Brien himself knows, such conclusions will not be pleasing to diplomats who envision what he scornfully calls "comprehensivist" solutions. But such solutions, he stresses, ignore the realities and history etched in the soil of that holy and bloody place, as well as in the wounded memories of the peoples that inhabit it.

O'Brien does believe that some tacit arrangements are possible with Syria and Jordan that would at least stabilize the area and give the Palestinians of the West Bank the possibility of a tolerable, if not fully satisfying, existence. Certainly, those arrangements would not satisfy the PLO, and might ultimately be undone; and it is for that reason, O'Brien says, that the end of Israel's siege is not in sight.