Fascism, to hear President Bush tell it, has been revived by Islamic militants. ''The terrorists are the heirs to fascism,'' he has said. ''They have the same will to power, the same disdain for the individual, the same mad global ambitions. And they will be dealt with in just the same way. Like all fascists, the terrorists cannot be appeased: they must be defeated.''

In this statement, Bush laid out his checklist for what constitutes fascism. Such checklists are required because fascism -- unlike Communism, socialism, capitalism or conservatism -- is a smear word more often used to brand one's foes than it is a descriptor used to shed light on them. Robert O. Paxton, a professor emeritus at Columbia University and the author of several books, including ''Vichy France,'' is not the first scholar to wade into a definitional and historical quagmire in order to answer the question, What is fascism? Indeed, his book ''The Anatomy of Fascism'' -- which doubles as a history and a sustained argument -- is not the most original study of the subject. But it is so fair, so thorough and, in the end, so convincing that it may well become the most authoritative.

Why should readers care about fascism? Paxton offers one answer at the outset. ''Fascism was the major political innovation of the 20th century, and the source of much of its pain.'' But in exploring how such uncouth nobodies as Hitler and Mussolini introduced what the Italian philosopher and historian Benedetto Croce described as an ''onagrocracy'' -- or ''government by braying asses'' -- he also hopes to enable us to recognize ''what the 21st century must avoid.''

''The Anatomy of Fascism'' is the work of a distinguished scholar who has sifted through the primary sources, the tomes and the trends in an effort to synthesize and even settle prior debates. His main emphasis is on Mussolini's Italy and Hitler's Germany, but in order to demonstrate why certain fascist movements were able to seize power while most remained marginal, he contrasts these ''successes'' with fascist sputterings in Britain, France, Hungary, Portugal, Spain and elsewhere.

World War I and the Bolshevik Revolution of 1917 contributed mightily to the advent of fascism. The war generated acute economic malaise, national humiliation and legions of restive veterans and unemployed youths who could be harnessed politically. The Bolshevik Revolution, but one symptom of the frustration with the old order, made conservative elites in Italy and Germany so fearful of Communism that anything -- even fascism -- came to seem preferable to a Marxist overthrow.

Still, Paxton retains an important capacity for incredulity. How on earth was it that Benito Mussolini, who won a mere 4,796 votes out of 315,165 in the 1919 election, could find himself appointed prime minister in 1922? The answer, Paxton makes clear, was not Mussolini's policy platform. ''They ask us what is our program,'' Mussolini said. ''Our program is simple. We want to govern Italy.'' Rather, it was the societal ills, the conservatives' fear of a Communist revolution, the paralysis of Italy's liberal constitutional order and the violence inflicted by fascist militia -- violence that made the state eager to co-opt the violent themselves.

How could Hitler, whose Nazi Party placed ninth in 1928 (with only 2.8 percent of the popular vote), soar to first in 1932 (with 37.2 percent)? In Germany, storm troopers intimidated enemies, Hitler himself delivered mesmerizing harangues and the Nazi Party became a catchall movement that appealed to those Germans from all classes who were disillusioned with the bankrupt mainstream parties.

But none of this was enough to bring about fascist rule. One of Paxton's main contributions is to focus less on the ''Duce myth'' and the ''Führer myth'' and more on the indispensable ''conservative complicities'' behind the fascist takeovers. Paxton debunks the consoling fiction that Mussolini and Hitler seized power. Rather, conservative elites desperate to subdue leftist populist movements ''normalized'' the fascists by inviting them to share power. It was the mob that flocked to fascism, but the elites who elevated it. ''At each fork in the road, they choose the antisocialist solution,'' Paxton writes. King Victor Emmanuel III responded to Mussolini's ''gigantic bluff,'' the Black Shirt march on Rome, not by imposing martial law but by offering him the prime ministership. And in 1933 it was the ambitious German Catholic aristocrat Franz Von Papen, believing he would be the one who gained power, who arranged the deal that gave Hitler the chancellorship.

Fascist movements and regimes are different from military dictatorships and authoritarian regimes. They seek not to exclude, but rather to enlist, the masses. They often collapse the distinction between the public and private sphere (eliminating the latter). In the words of Robert Ley, the head of the Nazi Labor Office, the only private individual who existed in Nazi Germany was someone asleep. And, crucially, their durability depends on their ability to remain in constant motion. It was this need to keep citizens intoxicated by fascism's dynamism that made Mussolini and Hitler see war as both desirable and necessary. ''War is to men,'' Mussolini insisted, ''as maternity is to women.''

Paxton leaves his readers with a working definition of fascism:

''A form of political behavior marked by obsessive preoccupation with community decline, humiliation or victimhood and by compensatory cults of unity, energy and purity, in which a mass-based party of committed nationalist militants, working in uneasy but effective collaboration with traditional elites, abandons democratic liberties and pursues with redemptive violence and without ethical or legal restraints goals of internal cleansing and external expansion.''

Fine-tuning definitions, however, is less important for the future than identifying and neutralizing fascist threats. This recognition will come, Paxton believes, ''not by checking the color of shirts'' but ''by understanding how past fascisms worked.'' We should ''not look for exact replicas, in which fascist veterans dust off their swastikas,'' he writes; nor should we look for hate crimes and extreme nationalist propaganda. Rather, we should address the conditions and the enablers -- political deadlocks in times of crises, and conservatives who want tougher allies and elicit support through nationalist and racist demagogy.

For every official American attempt to link Islamic terrorism to fascism, there is an anti-Bush protest that applies the fascist label to Washington's nationalist rhetoric, assault on civil liberties and warmaking. Paxton's study has made it no less likely that the label will be appropriated. But the lasting contribution of this splendid book is to remind us that fascism, if it returns, will do so not simply because of a rousing leader, but because of his timid accomplices.