The most entertaining "what if" story I know might have qualified for this collection of jeux d'esprit by various historians, but didn't.

It is Barbara Tuchman's account in "The Guns of August" of how, at the outbreak of World War I, a German battle cruiser eluded British pursuit, made its way up the straits to Istanbul and by its presence there determined (according to Tuchman) that Turkey would side with the Central Powers.

The story derives much of its amusement value from its perfect illustration of the besetting flaw of "Cleopatra's nose" speculation about the near-run things of history. When Pascal attributed the fate of imperial Rome to the length of Cleopatra's nose, he was of course parodying the tendency of "counter-factual" history to overload a single, almost trifling, cause with greater "effects" than it can plausibly bear.

It is implausible to the point of grotesquerie to suggest that Turkey's alignment in a great war pivoted on the fate of a single warship. The truth is that most historical episodes of real moment are, as Freud said of neuroses, "overdetermined": They usually flow from many causes, intricately interwoven and interrelated. So the game played in some of these essays, amusing as it is, cannot qualify as history proper.

Indeed, "what if" history can be an amusing exercise on a slow day. The "near miss" stories told in this collection include some lively ones:

"What if" the famous Indian maiden Pocahontas had not stepped in to save Capt. John Smith from beheading, presumably making possible (because of the strength of his leadership) Jamestown's survival through the "starving time"?

"What if" Fidel Castro, who was once spotted as a talented righthander by Alex Pompez, the New York Giants Caribbean scout, had become a U.S. big league baseball pitcher instead of a Cuban revolutionary?

"What if" Louis XVI's ill-starred flight to Varennes in June 1791 had succeeded, allowing the French king to set up a counterrevolutionary government in exile? Or "what if" Voltaire, the French philosopher, had moved to Pennsylvania in 1753, as he seems to have considered doing? Would the French Revolution have occurred without his iconoclastic needling?

We cannot, in the nature of the case, know the answer to these questions, though some historical speculations display considerable charm, verve and wit. Others included here, mysteriously, have nothing at all to do with the "for want of a horse" theme, and still others are shallow, pedestrian, pedantic, execrably written or otherwise disagreeable. The collection is, in short, a very mixed bag.

For instance, in his essay, "What Pocahontas Did," Joseph Ellis is grimly bent on converting a small historical tale of some charm into that no-nonsense, unillusioned form in which the 20th-century mind likes to parade its immunity to the romance and poetry of the past. Maybe you thought it was indeed the charm of the story that had made it stick in our memory and imagination. But no, silly. Thus saith Ellis: "It . . . has been catapulted into the forefront of our collective memory primarily because it satisfies some need to believe that white men have a powerful and passionate appeal to non-white women. What Pocahontas did is remembered . . . because it plays to the prevailing prejudices of white, Western, male culture."

Not that that is the extent of Ellis' scolding. We are reminded, lest we forget, that "Indians" is a misnomer for "Native Americans," and by the time Ellis had done with hectoring us, we feel almost ashamed of ever having found anything but racism in the story of Pocahontas.

Ivo Banac's essay "Sarajevo 1914" is done, thank goodness, with a somewhat lighter hand. Banac speculates with some gaiety on the blunders of Franz Ferdinand's driver that day when a royal assassination lit the fuse to the powder keg that blew up the old Europe.

As it happens, A.J.P. Taylor speculated on the same situation. Only he blamed World War I not on the stupidities that give Princip a fatal shot at the archduke, but on the imperial law that required his marriage to his adored Sophie to be morganatic. In consequence, Sarajevo, a place crawling with Bosnian nationalist conspirators and gunmen, happened to be one of the few places within the empire where Sophie could ride as the archduke's equal in an open car! (For want of a nail, indeed.)

One has only to read a string of such essays, even when they're as bright and amusing as Peter Gay's "A Gentile Science," to see that the possibilities of the "counter-factual" or Cleopatra's nose form are somewhat short of limitless.

That historical truth is far stranger than fiction, and what actually happened usually more interesting than the infinitude of imaginable alternatives, is of course a cliche'. It also happens to be a sound one. Despite the occasional witty moment, "For Want of a Horse" serves in the end to remind us why most historians, most of the time, invest most of their energies in detection and reconstruction, not speculation.