In Geoffrey Household's tense thriller, Rogue Male (1939), a lone English adventurer takes a pot shot at Hitler and then runs for his life. Few Germans were brave or reckless enough to resist the Fuhrer. Once Hitler's madness was obvious, however, the dilemma for German patriots was painful: to love the Fatherland yet desire the downfall of Nazism.

Had Hitler been assassinated, what sort of Germany might have emerged? Claus von Stauffenberg, the man who put the bomb in the briefcase, reportedly cried out, "Long live holy Germany!" in front of his executioners  hardly an ideal slogan for a modern democratic nation. In Michael Newton's view, von Stauffenberg was the "apotheosis of Teutonic manhood"; he wore a steel helmet even to his wedding. Few would dispute the justice of killing Hitler, yet von Stauffenberg is viewed as an ambivalent hero in Germany today; his dog-like devotion to Prussian codes of honour is reckoned to be anachronistic.

For most assassins, says Newton, the intended victim is always a "Hitler" of sorts and deserving of death. Violet Gibson, the Dublin-born daughter of a Conservative MP, shot Mussolini three times at close range in Rome in 1926. The Italian dictator escaped with merely a grazed nose. Was Gibson mad? She had intuited the danger of fascism early on, and for this at least her attempt to eliminate the Duce might lay claim to our admiration.

In Newton's superb history of conspiracy and political violence throughout the ages from 1865-1981, assassins and would-be assassins emerge as complex and often paranoid personalities, who came close at times to changing the fate of nations. Inevitably, assassination is a subject that attracts conspiracy theorists of one stripe or another. Half a century on, cliques and shadowy cabals are still believed to have manipulated "the truth" behind the Kennedy assassinations of the 1960s.

Hydra-headed conspiracy theories flourish in the absence of hard evidence. For all his avowed allegiance to communist Cuba, Lee Harvey Oswald was probably unattached to any "red" paymaster. The Illinois-born drifter James Early Ray, who murdered Martin Luther King, was seemingly another freelance operator. Not only "Martin Luther Coon" (as Ray called him), but all white liberals were in his sights that fateful day in Memphis is 1968 when he pulled the trigger on King. In Ray's corrosive sense of racial grievance and "anti-nigra" politics Newton sees an echo of an earlier American assassination.

Abraham Lincoln's murder in 1865 was the work of a deluded white supremacist and pro-Confederate drunkard named John Wilkes Booth, who likewise viewed the world solely through the lens of racial conflict. Lincoln, in Booth's distorted vision, was a money-grubbing Yankee hostile to the gracious suavities of the gallant south. The slave-holding south had been so utterly devastated by Union armies that it lost 20% of its white male population; yet Confederates like Booth managed to recast themselves as Christ-like victims who were exalted by defeat. His act of "tyrannicide" against the president was intended to change post-bellum America for the better.

From Lincoln to assassination attempts made on Queen Victoria and Ronald Reagan, Newton contemplates political violence in all its complexity. Assassinations were at their most frequent amid the moral and material ruins of post-first-world-war Europe. In 1921, an Ottoman leader was gunned down on the streets of Berlin in revenge for Turkey's genocidal slaughter of Armenians. The assassin, a young Armenian named Soghomon Tehlirian, had exacted justice on behalf of the voiceless dead, he said.

Other assassins have been less transparent in their motives. President Nixon's would-be killer, Arthur Bremer, was a "no-hoper" who scrounged for a living in 1970s Milwaukee, and appeared to relish a life of subterfuge. On the eve of his long-planned assassination, he ritualistically shaved off his hippy-length hair for a Mohican Taxi Driver look. Yet his ambition to shoot Nixon at close range ("Can't kill Nixy-boy if you ain't close to him") came to nothing. Instead, Bremer shot and paralysed the segregationist Alabama politician George C Wallace. His motives for doing so remain obscure. In Newton's view, Bremer exuded "narcissistic self-concern" rather than a discernable ideology. In contrast to the bomb-throwing, dagger-wielding anarchist assassins of 19th-century Europe, certainly, he was a lone wolf.

In recounting the lives of lesser-known assassins like Bremer, Newton risks providing a platform for their monomania. Why grant them any publicity? Yet the point is well made: by the late 20th century, shooting a US president had become no different from shooting, say, John Lennon or Andy Warhol, because politics had by then become a subdivision of stardom. Valerie Solanas, who tried to kill Warhol 1968, was another who preferred to be noticed and infamous rather than not noticed at all. Increasingly paranoid, she was at heart insane, writes Newton; yet her attempt on Warhol's life was a prototype for many future assassinations as it was wilfully devoid of ideology.

Age of Assassins, fortified with allusions to film and literature, is an unusual work of social history. If the writing is at times overdone ("the grey area of our pampered and inadequate pity"), the book has immense narrative verve and psychological fascination. I was gripped from start to finish.

Assassination did not begin in 1865 any more than sexual intercourse began in 1963. History is strewn with the corpses of leaders cut down by disgruntled rivals or wild-eyed fanatics. The Romans did it, the medievals did it, the early moderns did it. What makes the period 1865-1981 so different that it can be called "the age of assassins"? Michael Newton's answer, in this rich and haunting new history, is not the number of assassinations but the quality of the assassins.

The killers he writes about were, for the most part, neither rivals nor fanatics. They were unremarkable people, which is what makes them so peculiar. Often, their motives are hard to fathom: this is the age of political murder as gesture politics. Newton's story starts with John Wilkes Booth, an actor who killed a president in a theatre. It ends, appropriately, with John Hinckley, the would-be assassin of Ronald Reagan, a disgruntled fan aiming to kill a president who had been an actor in order to get the attention of an actress.

Booth, Lincoln's assassin, is the bridge between the ages: he was both a throwback to an earlier time and a harbinger of things to come. He hoped to emulate the assassins of antiquity, who killed tyrants rather than live as slaves, though he was doing it on behalf of the American South, which was built on slavery. Booth saw himself as a selfless martyr, yet he was also a drunken poseur and a fantasist. The theatrical flourish with which he shouted out, at the crucial moment, the Latin motto of the state of Virginia "Sic semper tyrannis" ("Thus always to tyrants") was, Newton writes, "both ludicrous and momentous". Lincoln's death was an epochal event and an exercise in futility. It did nothing to preserve the South from defeat and occupation. If anything it helped to accelerate it.

A theme of this book is the way that the consequences of assassination so often confound the expectations of the assassins. Sometimes, the effects are far greater than anticipated. Gavrilo Princip, the scruffy Serbian student and low-level conspirator who in 1914 killed Archduke Franz Ferdinand, the heir to the Hapsburg throne, wanted revenge for the indignities Austria had heaped on Serbia. Instead, he had to watch as the whole of Europe was consumed by a terrible war. This was so far removed from what Princip had intended that he acted as though it had nothing to do with him. But one of his co-conspirators, tasked with throwing bombs at the archduke's carriage, lamented as he saw the world go up in flames: "If I had foreseen what was to happen I should myself have sat down on the bombs so as to blow myself to bits."

More often, the assassins assume that their act will have a galvanising effect, when in fact the reaction is bemusement and contempt. Newton writes brilliantly about these botched assassinations and embarrassing failures. One was the attempted murder of the American industrialist and robber baron Henry Clay Frick in 1892 by the anarchist Alexander Berkman. The would-be assassin wanted to strike a blow for workers everywhere by killing one of their arch oppressors. But not only did Frick refuse to die  he survived being shot in the head  Berkman himself was attacked by a carpenter who happened to be in Frick's office at the time. Didn't this ordinary worker understand that the assassination was being undertaken on his behalf? No, he simply assumed Berkman was deranged and knocked him out with a hammer.

Many of the assassins in this book had a view of the public significance of their deeds that totally misjudged the public. Another anarchist, Leon Czolgosz, murdered the American President McKinley in 1901 in a gesture designed to expose the sham of American democracy. But if McKinley was really just a puppet, then removing him would change nothing. American democracy simply found another figurehead, the hugely popular Teddy Roosevelt, and carried on as before. Killing democratic politicians did not shake Americans' faith in democracy: they identified with the victim, not the perpetrator.

Because these crimes often seemed so senseless at face value, it was tempting to think that the true explanation must be hidden beneath the surface. Assassinations did not produce political change but they did produce plenty of conspiracy theories. Czolgosz, it was assumed, could not have been acting on his own; he must be part of some sinister anarchist network. But there was no conspiracy. Czolgosz was an impressionable loner who had attended a lecture by the anarchist intellectual Emma Goldman and taken her at her word: he decided to act for himself.

Czolgosz was in many ways a forerunner of the later, better known assassins of the 20th century, including another impressionable loner, Lee Harvey Oswald, the killer of JFK. We still find it very hard to believe that such a momentous act as killing a president could be undertaken by someone so insignificant. He must have been working for others. But the big difference, as Newton points out, is that where in earlier assassinations it was the government that had imagined a conspiracy, now government itself came to be seen as a conspirator. Thanks to films such as The Manchurian Candidate (1962), it was assumed that all political murders must be inside jobs. "Suspicions had shifted from anarchist cabals, reds, Roman Catholics or Jews to Washington DC." The suspicion lingers to this day.

This is an impressionistic book, built up of stories and observations rather than straight political analysis. Newton is very good on the human side of assassination and he always sees it from both sides, finding links between the hapless victims and the hapless perpetrators. He writes expertly about films and the pop cultural context for these acts. This human approach is less convincing when assassination needs to be explained in its political context, as for example in Weimar Germany, where routine political killing did contribute to the destabilisation of democracy, as is happening in Iraq and Afghanistan today. Here the killers know exactly what they are doing: the network of violence is all too real. The book rather skips over this side of assassination. The miserable European story of the interwar period doesn't really belong in the age of assassins: it is part of a longer and grislier tradition.

Newton's true subjects are the losers and misfits who stumble across assassination as if by chance  such as Arthur Bremer, the would-be killer of the southern demagogue George Wallace. Newton describes Bremer as "part Mersault, part Molesworth", a man-child all at sea in the social and sexual upheavals of late 1960s and early 70s America. The assassin is as confused as anyone as to why he did it: "My penis made me," he explains in exasperation. Or there is the petty thief and failed pornographer, James Earl Ray, who ended up assassinating Martin Luther King. Later, Ray pleaded innocence of the crime, claiming that a bum like him couldn't have done anything so momentous. "Assassination is out of my league," he protested. That's what made him so typical of the age of assassins.

 David Runciman's Political Hypocrisy is published by Princeton.