What makes a great leader? The ability to inspire others even as you unsettle them - a quality Tony Blair shares with the man who unified Germany, Otto von Bismarck, says Brendan Simms.

Ever since antiquity men have turned to history for lessons in leadership.

And things are no different today. At the most mundane level, aspiring business people the world over are increasingly turning to the great leaders of the past for guidance. Recent self-help manuals, with titles such as Elizabeth I CEO: Strategic Lessons from the Leader who Built an Empire; Nothing to Fear: Lessons in Leadership from FDR; and Alexander the Great's Art of Strategy: Lessons from the Great Empire Builder, speak for themselves.

In the exalted world of high politics, the search for historical invigoration is even more pronounced. Take, for example, Niall Ferguson's Empire: How Britain Made the Modern World, the US edition of which promises "lessons in global leadership". Likewise, strategic historian Eliot Cohen found his latest tome commended by US President George W. Bush - large numbers of copies were ordered for the White House after September 11 2001.

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Cohen's account of the leadership skills of president Abraham Lincoln, Winston Churchill and the first Israeli president, Ben Gurion, would - it was hoped - inspire similar qualities in Bush Jnr as he confronted the challenge of 21st-century terrorism.

Of all the exemplars of leadership, Churchill has been the most enduring, as the success of two recent biographies by Geoffrey Best and the late Roy Jenkins attests. The prescience with which he foresaw the emergence of the Nazi threat, the resilience with which he responded to early reverses in the war and the emotional intelligence with which he related to the British people make him an irresistible model for politicians today. It should therefore come as no surprise to find that prime minister Tony Blair's strong stance against Saddam Hussein was cast in explicitly Churchillian terms. Indeed, Blair implicitly suggested the comparison himself when he drew a parallel with the 1930s in his remarkable speech before parliament on the eve of the Iraq war.

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There is no doubt that Blair showed historic qualities of leadership throughout the Iraq crisis. Globally, he leveraged an important but modest military contribution and a seat on the United Nations Security Council seat to help persuade Bush to restart the Middle East peace process and bring the issue of Iraq back to the UN one last time. In Europe, Blair freed Britain from being a minority of one, in a group of three dominated by France and Germany, to lead nine European states more sympathetic to US intentions. Domestically, Blair kept the lid on parliamentary opposition through a combination of arm-twisting, rhetorical sleights of hand, and sheer force of argument.

Nor was this the first time that Blair had shown such decisive leadership.

It was manifested in his decision to enter talks with Sinn Fein without prior decommissioning, and in his persuasion of the Ulster Unionists to keep talking on the basis of promises that were later not so much disregarded as overtaken. The result was the Good Friday agreement of 1998 at which Blair famously spoke of feeling the "hand of history" on his shoulder. But perhaps the most remarkable example of Blair's leadership was his role in the defeat of President Slobodan Milosevic during the Kosovo war of 1999, when he kept his head while all around were losing theirs. All this contrasted with the timidity of his Conservative predecessors, none of whom were bad or stupid men.

To query the comparison with Churchill is therefore not to doubt the prime minister's ability to rise to a challenge similar to that of the 1930s. Nor is it to query whether Blair has Winstonian quantities of charisma. Nor even is it to deny that the prime minister showed Churchillian goodwill in his studied disavowal of triumphalism in victory. The distinction lies elsewhere: Churchill led his people once they had seen that there was no alternative; over Iraq, Blair led the British people, Whitehall and the armed services where they had never intended to go. In that sense, his achievement was the greater; it was certainly quite different. After all, leadership cannot be abstracted from context. As Marx famously reminds us, men may make their own history, but they do not do so in circumstances of their own choosing.

To find a parallel in creatively destructive, unexpected and perhaps unplanned leadership, we must turn to the Prussian and later German chancellor Otto von Bismarck, the architect of German unification in 1871.

At first sight, the comparison seems odd, even perverse. Bismarck was a staunch conservative; Blair is a Social Democrat. Bismarck saw himself as a conservative realist, a mere "navigator on the stream of time"; Blair sees himself as a radical reformer: he favours the rhetoric of possibility over that of limitation. Bismarck was a self-conscious apostle of realpolitik; Blair is the avatar of the doctrine of international community by which the world community is obliged to carry out humanitarian interventions, even where these conflict with state sovereignty.

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But in terms of leadership, the parallels are instructive. Both men transcended their origins. Bismarck moved from being a defender of a narrow conservative Prussian aristocratic interest to the hero of the liberal nationalist project, which meant the end of the Prussia he had sworn to defend. Likewise, Blair has travelled far from his roots as a campaigner for nuclear disarmament to linchpin of a shaky international coalition to effect "regime change" in Iraq. Both men looked for support across party lines. Both claimed, not entirely implausibly, to have remained consistent throughout. Neither, however, was ever trusted by his "core constituency" again. Like many great leaders, both men unsettled their following as much as they inspired it.

Both men had to contend with fractious parliaments and publics. Before German unification, Bismarck was tormented by the liberal majority in the Prussian assembly - the Landtag - which refused to pass his budgets. Blair confronts profound parliamentary scepticism on the wisdom and morality of the invasion of Iraq. More generally, he faces hostility on a range of domestic issues, especially taxation, from a public wary of the costs of messianic visions. They fear that whenever Blair feels the hand of history on his shoulder, they are about to feel his hand in their pockets.

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Above all, both men embarked on great integrative projects. Bismarck united Germany, perhaps without at first intending to. Blair has recast Great Britain through various devolutionary measures, in order to preserve it.

But the prime minister's ambitions seem to go much further: he wishes to play a central role in the unification of Europe and probably wishes to lead it. There is no other way of explaining his indulgence of the euro or his advocacy of greater defence integration in the teeth of transatlantic hostility. Whether he succeeds will depend on his ability to carry the Bismarckian model of leadership to its logical conclusion.

For the project of European integration is now at roughly the same stage as that of German unification by 1860. Ever since the Customs Union - or Zollverein - of 1834, economic integration had been proceeding apace; but political unification continued to founder on the objections of smaller German states to surrendering their independence, and especially on the refusal of Austria to allow itself to be sidelined by Prussia and ejected from Germany. What finally persuaded them to pool their sovereignty was the manifest failure of the German Confederation, a loose and congenial political commonwealth, to provide security against suspected French aggression. The confederation fluffed its challenges during the revolution of 1830 and the Rhine crisis of 1840, so Bismarck was able to sell unity as the only way for southern and western Germany to guard against the pretensions of Napoleon III.

So it is with the European Union. Here, too, the project of economic integration via the single market and single currency is well advanced.

Here, too, political and military integration has repeatedly stalled despite sustained efforts: the crises in the former Yugoslavia did not - as the much-lampooned foreign minister of Luxemburg suggested - prove the "Hour of Europe". The vaunted European defence identity was exposed as the very model of a modern chocolate soldier; swathed in a gaudy multilateralist wrapper, saccharine-sweet on the principles of consensus, but flaky when put to the test. The remedy, the greater defence cooperation announced at St Malo in 1998, collapsed most recently over Iraq. One can only speculate on what happens next. Will Blair lead and unify Europe by turning the next crisis into a war of European unification? Will he have to sideline the French as surely as Bismarck crushed the Austrians, and reconcile them afterwards?

Such leadership can only be legitimated through success. The Prussian Landtag was awed and beguiled by Bismarck into passing an "indemnity law", which retrospectively validated his actions. Whether Parliament will be so grateful and forgiving of Blair's leadership remains unclear. If it passes a new version of the indemnity law, Blair will be free to meet and even create new challenges. In that case, historians may say that the unification of Europe was brought about not by economic convergence, consensual resolutions and constitutional conventions, but by blood and iron. And they may well be reminded of Oliver Cromwell's dictum on leadership: "He goeth furthest who knows not where he is going."

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Brendan Simms is lecturer in international relations, University of Cambridge and fellow in history at Peterhouse, Cambridge.