If George III is Britain’s most misunderstood monarch, he is also one of the best known, thanks to Alan Bennett’s play The Madness of George III, which served as the basis for Nicholas Hytner’s much better film adaptation (titled The Madness of King George, allegedly to reassure any Americans fearing they might have missed two prequels). A smash hit with public and critics alike, the film garnered a sheaf of awards, including an Oscar and three Oscar nominations. Now, thirty years after the premiere of Bennett’s play, George may become Britain’s best-understood monarch, thanks to this impressive new biography.

It is unashamedly revisionist. The scene is set with a ripe selection of damning verdicts on King George, featuring everyone from Thomas Jefferson to Lin-Manuel Miranda (creator of the musical Hamilton), the most witty being E C Bentley’s clerihew ‘George the Third/Should never have occurred,/One can only wonder/At so grotesque a blunder’ and the most ignorant being J H Plumb’s equation of him with King John as ‘one of England’s most disastrous kings’. Roberts’s riposte to these and similar denunciations is a stark rebuttal: ‘every single word quoted above about George III is completely wrong.’ No prisoners are taken, as one detractor after another is skewered: Whig and Tory, left and right, British and American. Special punishment is meted out to George’s contemporary Horace Walpole, whose falsehoods and distortions were driven by ‘personal resentment, political opposition and preternatural levels of malice’.

Instead, we are given ‘the true story … written without malice or sycophancy for “all unprejudiced persons” to judge George objectively by the facts’. Agonising about what constitutes objectivity is not for Roberts. His George was a model patriot king, defender of the constitution, protector of liberty, upholder of the law, enlightened patron of scholarship, progressive agriculturalist and devout Anglican. At home, he was an exemplary family man, devoted to his children (not that most of them returned the compliment) and a loving husband, probably the only Hanoverian king never to commit adultery. If this rehabilitation seems a bit overdone at times, it is generally convincing. Less compelling is the lack of understanding shown for what motivated George’s numerous enemies, especially Jefferson, Thomas Paine, Benjamin Franklin and Henry Fox, who too often come across as pantomime villains.

It was George’s bad luck that he should have succeeded his grandfather in 1760, just as the Seven Years’ War was entering its final stage. Receiving no credit for previous successes, he was pilloried for accepting peace terms condemned as inadequate. As if that miserable start were not enough, he was also left with the intractable problem of restoring stability to finances wrecked by war. In the opinion of George, his ministers and most Britons, it was not unreasonable to ask the inhabitants of the thirteen American colonies to shoulder a fair share of the enormous burden imposed by the needs of imperial defence. And so began, with the passing of the Stamp Act in 1765, nearly two decades of conflict, climaxing with the Declaration of Independence in 1776 and ending with the recognition of the United States of America in 1783.

This was the big event of George’s reign and it is the big topic of this biography, consuming exactly half of the twenty-eight chapters. Roberts’s account is masterly, combining a compelling narrative – one has to keep turning the pages even though one knows the outcome – with analysis that is both cogent and incisive. He appears to have read everything that is in the mainstream and much that isn’t, including a wide range of archival sources. Given that only three years have passed since the publication of his biography of Churchill, this suggests a truly Stakhanovite work rate. Like the American Revolutionary War itself, the perspective is global. As he observes, after the French joined the fray, ‘trying to quell the American rebellion took second place to defeating France’, at least until the Yorktown debacle of October 1781 returned the American theatre of operations to centre stage.

However, this is not an account of American liberation that will commend itself to all American readers, especially if they are of a patriotic disposition. In particular, Roberts’s dissection of the Declaration of Independence may ruffle feathers, as he dismisses almost all the charges levelled against the king, before concluding that ‘in terms of genuine as opposed to theoretical liberties, Britain was well ahead of North America’. He also accuses slave-owning Jefferson of committing ‘a staggering piece of hypocrisy’ by including in his original draft the charge that George was responsible for the slave trade, and reminds his readers that forty-one of the fifty-six signatories to the Declaration owned slaves at some point in their lives.

Roberts clearly enjoys himself while recounting episodes which reveal that it was not only devotion to the promotion of ‘life, liberty and the pursuit of happiness’ that lay behind the colonists’ alienation from Britain. Among other bones of contention was the ‘Proclamation Line’ imposed by Lord Halifax in 1763. By creating a gigantic Native American reserve west of the Appalachians, it destroyed the hopes of voracious land speculators, including several of the Founding Fathers. Also deemed detrimental to American interests was Lord Mansfield’s judgment in Sommersett v Steuart in 1772, which ruled that slavery was contrary to English common law and precluded by Magna Carta. Two years later, the Quebec Act, which allowed French-Canadians to retain their language, culture and religion without interference from London, introduced a toxic anti-Catholic ingredient into the rapidly fermenting mix. Roberts supplies several other examples of material self-interest and prejudice supporting – or perhaps even driving – ideological conviction.

Roberts is quite clear about the bottom line in the divorce. It was certainly not about taxation: not only did the average American in 1770 pay a tiny fraction of what his British equivalent paid in direct taxes, but everything he did pay stayed in America. The essential issues were sovereignty, independence and self-government. By the middle of the 18th century, the Americans had developed so far in terms of prosperity, literacy, social organisation and political participation that they could no longer endure a colonial relationship. But to justify their illegal secession they had to demonise their legitimate sovereign by portraying him as a despotic tyrant.

Due attention is paid to the horrors of George’s recurring mental affliction. Roberts argues convincingly that this was not a case of porphyria, as the mother-and-son team of Ida Macalpine and Richard Hunter stated in their celebrated book George III and the Mad-Business in 1969. In reality, George suffered from hypomania, or bipolar affective disorder, succumbing for the first time in 1765 and most famously in 1788–9, which precipitated the Regency Crisis (and provided Bennett with the material for his play). In an important sense, the less serious attack of 1804 was also the worst, for it brought a breakdown in what had been a happy marriage. By that time, George was a pitiable figure, not least because all his numerous sons had gone off the rails, none more spectacularly than the ghastly Prince of Wales.

George died in January 1820, aged eighty-one, blind, deaf, demented, racked by rheumatic pain, remembered by his subjects only to be derided. He has had to wait two centuries for rehabilitation, but it has come at last. Roberts has got deep inside George and his world and has found a man of many sterling qualities, certainly ‘more sinned against than sinning’, to quote from King Lear (the reading of which, amazingly, was part of his therapy during one of his bipolar episodes). When his adored father, Frederick, Prince of Wales, died unexpectedly in 1751, aged only forty-four, he left his twelve-year-old son George a political testament in which he wrote, ‘I shall have no regret never to have worn the Crown, if you but fill it worthily.’ As Roberts writes in the closing sentence of this tremendous book, ‘George III more than filled the role of King of Great Britain worthily; he filled it nobly.’