To be viewed with contempt is an occupational hazard for politicians everywhere, but few statesmen have been as reviled as Robert Stewart, Viscount Castlereagh (1769-1822). He spent many years in England's Parliament managing the business of legislation for various Tory governments and served as the British foreign secretary during the Napoleonic wars and their aftermath. He was an inept public speaker—one critic said that he favored "every tax but syntax"—but he was a virtuoso at parliamentary maneuvering and a diplomat of skill and integrity. Not that such qualities won him many admirers.

Irish-born, Castlereagh helped secure the Act of Union that incorporated Ireland into the United Kingdom, for which some of his Irish compatriots, eager for independence, branded him a traitor. He pushed through Parliament laws restricting popular agitation and public meetings in order to keep protests from turning into a British version of the French Revolution. For his efforts, he was called an enemy of freedom. As foreign secretary, he became the architect, with Austria's Prince Metternich, of the post-Napoleonic settlement at the Congress of Vienna. Here he aimed at a stable balance of power after 15 years of war, setting aside the demands of various nationalities for their own states and giving dominion to the major powers. Critics accordingly damned him as a handmaiden of despots, a foe of liberal nationalistic aspirations. In a poem titled "The Mask of Anarchy" (1819), Shelley depicted Castlereagh as the grim reaper feeding his dogs with human hearts. Lord Byron wrote a notorious mock-epitaph: "Posterity will ne'er survey / a Nobler grave than this: / Here lie the bones of Castlereagh / Stop, traveler, and piss."

Castlereagh: A Life

By John Bew
Oxford, 722 pages, $39.95

An 1819 cartoon by George Cruikshank depicting Castlereagh (in white pants, crying 'Och! by the powers! and I don't like the looks of him at all! at all!') and others fleeing a caricature of the reformers then agitating for universal suffrage. Private Collection/The Bridgeman Art Library

However unfair such attacks could be, the diabolical image stuck. Historians of a liberal or Marxist bent endorsed it, giving his reactionary credentials an iconic status. Still, many scholars and statesman—not least, in recent times, Henry Kissinger—have come to a more nuanced view. They praise Castlereagh for helping to transform European politics from competitive struggle to peaceful coexistence, ushering in the long peace that allowed bourgeois society to flourish in Europe. Even in domestic affairs, Castlereagh and his Tory colleagues have lost their ogre aspect. That England did not become France meant that its political give-and-take produced reform rather than bloody upheaval. On closer inspection, the monster can appear to be rather moderate.

This is very much the view of John Bew, a lecturer in war studies at King's College, London. His finely etched portrait, "Castlereagh: A Life," is a readable work of scholarship that, among much else, makes a point of describing the Irish background that helped to form Castlerereagh's outlook. A neglected and often misunderstood Ulster milieu, Mr. Bew argues, grounded Castlereagh in an Enlightenment tradition of civic humanism; he wasn't, in short, some backwoods reactionary bent on thwarting liberal hope.

Despite his aristocratic title, Castlereagh sprang from an upwardly mobile family of Presbyterian merchants-turned-gentry in Northern Ireland. Belfast—whose prosperity made it a kind of Irish Liverpool—supported a vibrant intellectual culture. An Irish variation of the Scottish Enlightenment, Mr. Bew says, formed its values. The ideas of Francis Hutcheson played a defining role, especialy the claim that man's moral sense tended toward moderation and civic humanism—not the Hobbesian violence and viciousness that would require a repressive state. Castlereagh's father, a member of the Irish Parliament, knew some of Ireland's most sophisticated thinkers. Thus young Robert grew up amid Enlightenment texts and conversation. His early sympathies lay with the Americans fighting for independence, but he also had family ties with the British elite that provided a foothold in London.

Robert Stewart, Viscount Castlereagh (1769-1822) The Bridgeman Art Library

Irishmen took the American Revolution as a chance to secure increasing autonomy from Britain, but sectarian conflict meant instability. Catholics and Protestants outside the established church lacked political rights, leaving politics to a small minority. The French Revolution brought the prospect of unrest and violence. Britain's prime minister, William Pitt, found a solution by combining an extension of political rights to Catholics with a political union that would incorporate Ireland into Great Britain, with full representation in Parliament.

As a fairly new member of Ireland's Parliament, Castlereagh embraced Pitt's plan. He saw it as the best way to secure what he thought of as ordered liberty. For Castlereagh, Mr. Bew suggests, ordered liberty was a core political idea because order made possible the civic humanism that his Enlightenment background so valued.

In 1798, Pitt appointed Castlereagh chief secretary of Ireland, a position whose responsibilities involved managing legislation in Dublin and administering the Irish government. No Irishman had held the post before. Castlereagh guided the Act of Union through Ireland's Parliament—with bribery smoothing the way. Unfortunately, Catholic emancipation in Ireland didn't accompany the union, as had been promised. George III blocked it. Securing the union made Castlereagh one of Pitt's most prominent protégés, but his name, in Ireland, almost rivaled Oliver Cromwell's for hatefulness.

Castlereagh soon moved to London, becoming a member of Parliament and a rising star. Within a year of Pitt's death in 1806, he was named war secretary, joining a team of rivals in the new Tory government. For the fight against Napoleon, he strengthened the army and appointed an old friend—the future Duke of Wellington—to command British forces in Portugal. But failure to change the war's course soon brought trouble. George Canning, the foreign secretary, agitated against him, pleading behind closed doors to have Castlereagh removed from office. When Canning's efforts came to light, Castlereagh was furious and challenged him to a duel. Both men resigned their positions, and on Sept. 21, 1809, they met at Putney Heath outside London. On the first exchange of fire, no one was hit. Castlereagh demanded another try and, in the second exchange, put a bullet through Canning's thigh.

As Canning recovered physically, Castlereagh recovered politically, returning to office as foreign secretary in 1812. It was an opportune time. With Lord Liverpool as prime minister, Britain put new effort into the war just as Napoleon's fortunes crumbled with defeats in Russia and Spain. The key moment came in January 1814 when Castlereagh became the first foreign secretary to leave the realm and conduct negotiations directly. His arrival at allied headquarters decisively changed the dynamic, allowing Britain to protect its interests and giving him the chance to frame the terms of surrender that were offered to Napoleon so that, when the French general rejected them (as he did), the coalition would be tighter than before.

Victory soon brought the allies to Paris. There Castlereagh negotiated a generous peace with France, now under a restored monarchy. Napoleon, who would have imposed crushing terms on a defeated foe, thought him weak. But Castlereagh (with Metternich) was intent on a balance of power that would contain rather than encourage rivalry in Europe. For this, France was needed as a member of the community of nations, one that would serve as a counterweight to Russia. Nationalist aspirations—in Poland and Italy and elsewhere—would have to wait. As Mr. Bew shows us in detail, hammering out a settlement took more than a little brinkmanship with Czar Alexander.

Before the final agreement—and Napoleon's last battle at Waterloo—Castlereagh had returned to London. There he led the government side in the House of Commons. Liverpool, who sat in the House of Lords, desperately needed him, since peace with France had unleashed political opposition fueled by demands for lower taxes. Castlereagh bore the brunt of it, along with managing foreign policy as the alliance system frayed.

When unexpectedly cheered by an Irish crowd in 1821, Castlereagh joked that, compared with popularity, "unpopularity is the more convenient and gentlemanlike condition." Indeed, the stoicism of his public manner left those first meeting him surprised by his private charm. Tragedy came in 1822, when Castlereagh broke down from the strain of his government service and, in a fit of paranoia—he imagined plots against him and imminent arrest—killed himself by opening his carotid artery with a penknife.

His achievements lived on. As the liberal statesman John Morley said decades later, time made Castlereagh bigger. Mr. Bew impressively adds yet new dimensions to the man, showing how Castlereagh's practical statesmanship allowed Enlightenment ideals to serve conservative ends.

—Mr. Hay, a historian at Mississippi State University, is writing a life of Lord Liverpool, British prime minister from 1812 to 1827.