Geoffrey Wheatcroft is an English journalist and author, whose books include Yo, Blair! (Politico’s Publishing, 2007), The Strange Death of Tory England (Allen Lane, 2005) and The Controversy of Zion (PerseusBooks, 1997), which won a National Jewish Book Award .

For God’s sake do not drag me into another war,” said the Reverend Sydney Smith in 1823.

I am sorry for the Spaniards—I am sorry for the Greeks—I deplore the fate of the Jews; the people of the Sandwich Islands are groaning under the most detestable tyranny; Bagdad is oppressed . . . Thibet is not comfortable. . . . The world is bursting with sin and sorrow. . . . Am I . . . to be eternally raising fleets and armies to make all men good and happy?

That witty and humane clergyman had lived through a time of troubles: the American rebellion followed by the French Revolution and the Napoleonic Wars, strife over more than twenty years at terrible cost to the inhabitants of the countries where war was waged, from Spain to Russia. Although the price paid by England was slight by comparison, at least in human life if not in gold, those conflicts made the very question of war—whether and why it should be waged—as lively a topic in England as it is today in America, or should be.

This question is comparatively new. Once upon a time, the king declared war, his army fought it, the country paid for it, and that was that: no political problem arose. But the English civil war of the 1640s, the Glorious Revolution and the beginning of constitutional government meant that for the first time there could be open debate between a “Party of War” and a “Party of Peace.”

In the first years of the eighteenth century, the War of the Spanish Succession saw Jonathan Swift publish The Conduct of the Allies, denouncing the conflict, the way it was waged by the government in London, and in particular by the Duke of Marlborough, the general whose greatest victory gave its name to Blenheim Palace north of Oxford, where his descendant Winston Churchill would be born. Then, with the French Revolution and the Napoleonic Wars, Englishmen for the first time could openly salute their country’s declared enemy. (Some might say that the English critic William Hazlitt’s adulation of Napoleon began another tradition: writers and savants fawning over distant tyrants.) And although in the aftermath of war there may have been an aversion to taking part in other people’s conflicts, a new and potentially dangerous doctrine could be seen in the making.

It was not until 1997 that Robin Cook, Tony Blair’s first foreign secretary, said that “our foreign policy must have an ethical dimension,” but he was only formulating a concept that found its origins in the early nineteenth century if not before. The English had persuaded themselves that their foreign policy was, and if necessary their wars were, justified by moral purposes, for the greater good of other countries where liberty and enlightenment should be spread.

One of Cook’s predecessors is George Canning, foreign secretary in the 1820s, who proclaimed himself “an enthusiast for national independence” and supported the new South American republics which had rebelled against Spain. As foreign secretary three times, Lord Palmerston called himself Canning’s heir, supported the forcible restoration of constitutional government in Portugal, acclaimed the new liberal tide throughout Europe, and blusteringly insisted that “a British subject, in whatever land he may be, shall feel confident that the watchful eye and the strong arm of England will protect him against injustice and wrong.” This coincided with the rise of a newspaper-reading middle class that could be swayed by moral or emotional arguments.

And it was this new zeal for what would later be called liberal interventionism that Smith had in mind with his sardonic words. For the best part of two hundred years since, his successors have tried to argue against the use of force to make all men good, sometimes with success but too often without...