Was it a civil war twice over? Not only did the “war between the states” divide the American people, it sundered the larger English-speaking community stretching across the Atlantic. The conflict was followed with consuming interest by the British, it affected them directly, many of them fought in it — and it split them into two camps, just as it did the Americans. Now that Americans are taught that the war was a noble conflict waged by Lincoln and the forces of light against misguided and contumacious Southerners, it’s especially valuable to be reminded that this was far from how all the English saw it at the time. To be sure, almost no Englishman defended slavery, long since abolished in the British Empire. The British edition of “Uncle Tom’s Cabin” had sold an astonishing million copies, three times its American sales, and the Royal Navy waged a long campaign against the slave trade: during Prime Minister Gordon Brown’s visit to the White House in March 2009, President Obama was presented with a pen holder carved from the wood of one of the ships that conducted that campaign. But while some English politicians, like the radical John Bright and the Whig Duke of Argyll, ardently supported the North, plenty sided with the Confederacy. They even included W. E. Gladstone, on his long journey from youthful Tory to “the people’s William,” adored by the masses in his later years. Apart from sympathy with the underdog, many Englishmen believed that the South had a just claim of national self-determination. As Obama remembered to say at Buckingham Palace recently, a large part of the American population claims ancestry from British immigrants, great numbers of them arriving throughout the 19th century. Plenty of those took part in the war, and they were joined by more volunteers who came just for the fight, on one side or the other. The extraordinary cast portrayed in “A World on Fire,” by Amanda Foreman — who is also the author of “Georgiana: Duchess of Devonshire” — extends from men who fled England to escape poverty to aristocratic Union officers like Major John Fitzroy de Courcy, later Lord Kingsale, a veteran of the Crimea, not to mention Colonel Sir Percy Wyndham, a soldier of fortune whose knighthood was actually Italian. Some, like the Welshman Henry Morton Stanley, even managed to fight for both sides. Then there were the reporters, like Frank Vizetelly of The Illustrated London News and, most notably, William Howard Russell of The Times of London, who had become famous covering the Crimean War and reporting on the activities of Florence Nightingale. (In an odd conjunction, Foreman says that “Russell was the ideal choice. . . . Overeating and excessive drinking were his chief vices.” This is sometimes said of journalists, but rarely by way of commendation.) Such eyewitnesses provide a wealth of vivid description — and here is the one drawback of this thoroughly researched and well-written but exceedingly long book. The presence of so many Englishmen means that Foreman can too easily slip away from “Britain’s crucial role” to a general history of the war and its every battle. But there truly is no shortage of such histories, and we have all often enough vicariously supped full of the horrors of Antietam and Fredericksburg. What for American readers will be a more riveting — because unfamiliar — tale comes whenever Foreman turns from the patriotic gore to her true subject of the British and the war. While guns blazed, another battle was being waged, for English hearts and minds, at both the elite and popular levels. From Fort Sumter on, the London government was in a quandary, and so was Lord Lyons, who had the bad luck to be sent as minister to Washington shortly before the war began (the British representative was not yet an ambassador, of whom there were then very few, although not just three, as Foreman thinks). Lyons carried out his difficult task with patience and courtesy. On the one hand, Southern politicians threatened that if London did not recognize the sovereignty of the Confederate States, the cotton trade would be cut off, driving England to economic collapse and revolution. On the other, the Union administration warned that such recognition could lead to war. In the event, London toyed with recognizing independence, and angered the North quite enough by acknowledging the South’s belligerent status. Both sides had agents hard at work in England. Charles Francis Adams, scion of a famous Boston dynasty, was sent as American minister to the Court of St. James’s. He did as well as he could, although it didn’t help that he hated small talk, drinking and dancing, and that, as his son Henry said, “he doesn’t like the bother and fuss of entertaining and managing people who can’t be reasoned with,” which might be considered a definition of any diplomat’s job. What nearly did take Washington and London to war was the principle of freedom of the seas. To make his case in London, Jefferson Davis dispatched two Confederate commissioners in November 1861 aboard the Trent, a British mail packet. But the electrifying news came that crewmen from the U.S.S. San Jacinto had boarded the ship near Cuba and seized the two. “Have these Yankees then gone completely crazy?” Friedrich Engels asked his colleague Karl Marx, who himself wrote a good deal about the Civil War. Taking “political prisoners” in this way, Engels thought, was “the clearest casus belli there can be. The fellows must be sheer fools to land themselves in war with England.” Despite this provocation, war did not follow. Other Confederate envoys reached London, and many Englishmen remained susceptible to the Southern claim. An unlikely British best seller was “The American Union,” written by James Spence, a Liverpool businessman who had traveled widely in America. Although he was scarcely disinterested — Liverpool had prospered in the slave trade and then by cotton — he argued plausibly that North and South were so different that enforced union was futile. And he held, not so implausibly either, that since slavery was doomed in any case, it was better that it should be ended without violence. This was taken up by John Delane, the editor of The Times, who maintained that the war was a contest for Southern “independence” against Northern “empire.” Still the Union blockade of the South continued, and many English ships continued breaking it or trying to; Wilmington, N.C., to Bermuda was one favorite route. Meanwhile, the Confederate government clandestinely commissioned warships from English shipyards. Most famous of these was the Alabama, built by Laird & Sons. The intended purpose of the ship was obvious, as Adams’s Liverpool consul told him, and as the London government belatedly admitted. But the Alabama escaped from under official noses in July 1862 to begin a devastating career raiding Northern ships, to the fury of Washington. As if that rage weren’t enough, Lyons had to deal with the problem of British subjects caught up in the fighting. Both sides treated prisoners of war harshly. Of the 26,000 Confederate soldiers held over the course of the war at Camp Douglas near Chicago, more than 6,000 died, and at one point the prisoners there included 300 who claimed to be British subjects. They pleaded for Lyons’s intervention, but there was little he could do. One of the prisoners was the deplorable Stanley, who adroitly solved the problem by switching gray uniform for blue, unconcerned with politics: as he said, “there were no blackies in Wales.” A succession of Southern victories further encouraged English sympathy for the South. In late 1862 Lord Hartington, subsequently a cabinet minister, and nearly prime minister, visited both North and South (it was surprisingly easy to cross from one to the other), at first proclaiming his neutrality. But in Virginia he met Jefferson Davis, as well as the modest and agreeable Robert E. Lee, and was persuaded that the South was fighting virtuously for her rights. Hartington couldn’t pretend that blacks were flourishing, but then “they are not dirtier or more uncomfortable-looking than Irish laborers” (an unhappy comparison so soon after the great famine, and from a man whose family owned huge estates in Ireland). In its later stages, the war saw Southern terrorist conspiracies initiated from Canadian soil, which further inflamed the North. But English sympathy for the South lingered up until Lee surrendered at Appomattox in April 1865. Then, within days, came the shattering news that Lincoln had been assassinated. All at once, “newspapers that had routinely criticized the president during his lifetime,” Foreman writes, “rushed to praise him.” There were some wonderfully hypocritical about-faces, one from The Times, but best of all from Punch. Having just included Lincoln with Napoleon III in a gallery of April Fools, the magazine now hailed him as “a true-born king of men.” Not the least absorbing part of Foreman’s story comes after the war. Stanley was hired by The New York Herald and set off on his African journey to find Dr. Livingstone, before returning to England, a seat in Parliament and a knighthood. That fascinating figure Judah Benjamin, the Jewish lawyer who served as Confederate secretary of state, fled to London, where he became a barrister and published “Benjamin on Sales,” a commercial law textbook that made him rich. No American politician was now more vehemently Anglophobic than Senator Charles Sumner, who continued to denounce England, and whose verbal violence delayed a settlement of the Alabama dispute. His great rival, William Henry Seward, Lincoln’s secretary of state, also turned up the heat, demanding the Bahamas in recompense for the Alabama’s depredations, although he had further designs on Canada, as so many Americans did. In the end, the Alabama question was settled admirably, by jaw-jaw rather than war-war, as Churchill might have said, when an arbitration tribunal meeting in Geneva awarded large damages against Great Britain. The London government paid without complaint, inaugurating a period of comparative harmony, until Anglo-American war nearly broke out again in 1895 over an obscure Venezuelan boundary dispute. Altogether Foreman’s remarkable book should be a caution against one foolish phrase. A relationship, no doubt — but “special”? Geoffrey Wheatcroft’s books include “The Strange Death of Tory England” and “Yo, Blair!” He is writing a book about the reputation and posthumous cult of Winston Churchill.