It has done no favours to the modern reputation of King Frederick II of Prussia – ‘Frederick the Great’ – that Hitler, during the mad final days of the Third Reich, placed the monarch’s portrait above his desk in the Berlin bunker. For much of the period since, a powerful narrative – purveyed mainly, but not exclusively, by historians on the German Left – has traced a causal chain backwards from the Nazi catastrophe, via the belligerent imperialism of Kaiser Wilhelm II and the belligerent federalism of Bismarck, to the belligerent absolutism of Frederick the Great: the fons et origo of All That Went Wrong. For Frederick was the first German ruler to turn his miscellany of territorial possessions – Prussia was a third-rate country, if that, at the time of his accession at the age of twenty-eight in 1740 – into one of Europe’s great powers, and to do so by military force.

One of the most striking – indeed controversial – features of German historiography in the last decade has been the sound of the causal chains linking these episodes snapping one by one. The liberating historians, ironically but perhaps necessarily, have been foreign – indeed, mostly British. Christopher Clark’s brilliant history of Prussia, Iron Kingdom (2006), and still more his account of the origins of the First World War, The Sleepwalkers (2012), have caused a sensation in Germany, selling in their hundreds of thousands, not least because, with powerful archival scholarship and hard-edged argument, they cut through the links of responsibility that had made Germany and the house of Hohenzollern uniquely culpable for the descent into war in 1914. In so doing, they also severed the presumed connection between an alleged ‘Prussian despotism’ in the 18th and 19th centuries and Nazi tyranny in the 20th.

Tim Blanning’s superlative new biography of Frederick – ostensibly the least polemical of books – seems likely to have a similarly emancipatory effect. This is partly because Blanning places Frederick’s notoriously belligerent and opportunistic foreign policy within the context of the mid-18th-century European state system. This was a world in which all major powers used the threat (and often the reality) of force in pursuit of their territorial ambitions, and where the international alliance system was at least as competitive and volatile as anything in the decade before 1914. Frederick’s territorial ambitions – even his notorious seizure of Silesia (the most lucrative province of the Habsburg Monarchy) in 1740 – were not qualitatively different from those of other major European powers, Blanning argues; Frederick was just much bolder, more effective and, ultimately, luckier in realising them. Apportioning guilt among the various predators in the piranha tank of 18th-century European politics is – Blanning implies – a delusory and ultimately fruitless task.

What emerges, instead, from these pages is an almost sculptural, three-dimensional rendering of Frederick, one that enables its vast and protean subject to be viewed from a multiplicity of angles – as ruler of Brandenburg-Prussia for almost half a century, as military commander, amateur architect, poet, flautist, composer, librettist, law-reformer, and as atheist and Enlightenment savant (to name but a few) – while never losing sight of how each singular aspect relates to the whole. The result is a biography constantly alert to what Blanning calls the play of ‘light and dark’ in each area of Frederick’s actions: his failures and successes, his vices and virtues, and, in particular, the paradoxes and contradictions of his highly complicated – by turns attractive and repellent – personality. Blanning has produced a supremely nuanced account, abounding in novel assessments and insights, and one which – to its great credit – is destined to confound, in equal measure, Frederick’s parti pris admirers and detractors alike.

Central to Frederick’s formation was his powerfully reactive relationship with – and against – his domineering father, Frederick William I (who reigned from 1713 to 1740). Intermittently mentally unstable and prone to volcanic rages, Frederick William combined austere Lutheran piety, aggressive philistinism (he hated almost all forms of culture) and an obsessive pride in the Prussian army. He was also vigorously heterosexual.

He produced a son who was his polar opposite: a sceptic who came to despise all forms of Christianity (that ‘old metaphysical fiction’); an aesthete who became a distinguished patron (and practitioner) of music and the arts; a soldier who seemed, at least as a youth, bored by military life. He was also self-evidently homosexual in his attractions (an aspect of his personality which Blanning treats with a subtlety and frankness that has eluded most of Frederick’s earlier biographers). Frederick’s reaction against the purgatorial regime his father inflicted on him, Blanning contends, profoundly influenced the trajectory of his later achievements, both in the domestic sphere and on the battlefield.

Blanning argues persuasively that Frederick’s domestic rule can be justifiably described using the much-contested term ‘enlightened despotism’. Its ‘enlightened’ character was manifest in a multiplicity of forms: in Frederick’s abolition of torture within his realms; in his efforts to systematise Prussia’s laws into a single code; in his attempts to make Berlin a haven of freedom for philosophical thought and speculation, to the benefit of (among others) Rousseau, Kant and that professional ingrate Voltaire. By the end of Frederick’s reign, most of his subjects probably enjoyed a greater degree of religious and personal freedom than was available anywhere else in Europe – including, in religion at least, Britain. And while Blanning is alive to the occasional instances when Frederick acted with arbitrariness or severity, ‘in an ocean heaving with irrational cruelty, a sovereign who was merely severe stood out as an island of humanity’.

Blanning is particularly impressive in his richly detailed treatment of Frederick’s achievements (and limitations) in the field of culture. Here, too, he confounds the traditional claim that Frederick was in thrall to French culture, pointing out that, for all his admiration of the French language (he wrote and read almost no other), his tastes in opera were emphatically un-French. He despised the Francophile Gluck, for instance, and patronised German composers who set Italian libretti; and in architecture he preferred Roman and English models (the vast Neues Palais at Potsdam was inspired by Vanbrugh and Hawksmoor).

If much of what Frederick did and thought – in religion, culture and at court – can be seen as a reaction against his father, much of what Frederick achieved in the military sphere was, Blanning argues, an (ultimately successful) attempt to surpass the ‘Soldier King’. Nevertheless, it is as a military commander, where for two centuries the Prussian king’s reputation has stood highest, that Frederick comes in for some of Blanning’s sharpest censures. Frederick, he points out, lost almost as many battles as he won, and even where he was victorious, he was often saved from his own blunders by ‘the enterprise of his subordinates’ and the self-sacrifice of the rank and file. His ‘horrific mistakes’ as a commander, particularly in the Seven Years War (1756–63), ‘surely negate any claim to the genius rating claimed [for him] by his more enthusiastic admirers’.

Frederick’s more enthusiastic admirers may also have difficulty with that other aspect of their hero, which until recently was denied or discreetly passed over: his homosexuality. As Blanning makes clear, this was not some incidental aspect of his personality, kept quietly and furtively in the royal closet. While the degree to which Frederick was sexually active will necessarily remain unclear, he left the world in little doubt as to his sexual preferences when, at his accession, he put aside the wife he had earlier been forced to marry and set up his favourite, and probable lover, the Venetian polymath Count Francesco Algarotti, with a lucrative estate and a Prussian patent of nobility.

Such signals were given out with even greater clarity in Frederick’s private palace, Sanssouci – the rococo jewel that he created near Potsdam – where the atmosphere was ‘homosocial, homoerotic, and probably homosexual too’. There, making ‘an unmistakable statement’, Frederick placed his finest purchase – a celebrated classical bronze believed to be of Antinous, the lover of the Emperor Hadrian – in an open-sided pavilion ‘in direct line of view from his study’ and adjacent to the vault built to contain his own remains.

Tim Blanning is that rarest of scholars, as deft in his command of government and grand strategy as he is in his handling of philosophy and opera, and is rightly regarded as one of Britain’s (indeed Europe’s) finest historians. This biography finds him at the height of his powers and offers major reassessments of almost every aspect of Frederick’s career. If, in the end, we are left in no doubt as to Frederick’s ‘greatness’, it is nevertheless a greatness that is mercurial, astonishingly multifaceted, and as complex in its flaws as in its qualities. Those who would father upon Frederick the ambitions and military adventures of later German leaders will go away disappointed. But it is not the least of Blanning’s achievements that Frederick – in life, such a sedulous avoider of paternity – is freed at last from posthumous fatherhood of those wayward and entirely suppositious heirs.