Éminence grise, a term invented in the 17th century to describe an individual working in the background as lieutenant and confidant of someone more overtly powerful, is still in use today; like most clichés, it remains topical because it encapsulates an enduring phenomenon. Many statesmen have been supported by an éminence grise: William of Orange by Hans William Bentinck, Metternich by Friedrich von Gentz, Churchill by Brendan Bracken – and there are many other instances.

Yet probably the most challenging Éminence Grise to interpret remains the original, the man who first gave rise to the term. François Leclerc du Tremblay, known under his religious name as Father Joseph, was the mentor, ally and trusted agent of Cardinal de Richelieu during the era in which he single-handedly guided the destinies of France and much of Europe. Those two very different churchmen forged an unbreakable alliance in pursuit of their shared vision of a renewed France as the dominant power in Europe.

François Leclerc du Tremblay was born in Paris on 4 November, 1577, eldest son of Jean Leclerc, Premier Président des Requêtes du Palais and former ambassador to Venice. To this paternal heritage of noblesse de robe his mother Marie de La Fayette brought the added prestige of the landed nobility and, for her son François, the title of Baron de Maffliers, the style under which he first made his way in the world.

The early life of François du Tremblay followed the classic career of a young French nobleman. He was given a first-class education in which he excelled at Latin and all the traditional learning then in fashion. He also became a proficient horseman and acquired polished manners that were to stand him in good stead all his life. He rounded off his education with a trip to Italy. On his return to France he was presented at court where he made a very favourable impression: Gabrielle d’Estrées, the mistress of Henri IV, called him ‘the Cicero of France and of his age’.

In 1597 he served in the army at the siege of Amiens, from which the Spanish were successfully dislodged. During the subsequent peace negotiations François accompanied the diplomat, a distant kinsman, sent by France to negotiate with Elizabeth I of England. Tremblay was delighted by the learned atmosphere of the Elizabethan court; engaging in erudite conversation in Latin about Erasmus and classical authors, and enjoying English drama in Shakespeare’s heyday.

Yet in the midst of so much worldliness François du Tremblay faithfully nursed an unworldly ambition: he was resolved to enter the Church. In 1599 he became a Capuchin novice, was ordained in 1604 and, now known as Father Joseph, embarked on a spectacular career as an evangelizer, reformer and ascetic, making many converts. His most intractable challenge was to undertake a reform of the abbey of Fontevrault, inhabited by high-born nuns with no wish to be reformed. One outcome, however, was that he became co-founder of a strictly ascetic order of contemplative nuns, the Daughters of Calvary.

During these endeavours Joseph encountered the young reforming bishop of the nearby diocese of Luçon, Armand Jean du Plessis, the first bishop in France to implement the decrees of the Council of Trent. This was a fateful meeting between Father Joseph and the future Cardinal de Richelieu. Soon they began to have conversations about politics. These revealed an apparently irreconcilable difference of outlook between them.

Strangely, for a man subsequently associated with a calculating Realpolitik, Father Joseph was obsessed by the medieval concept of reactivating the Crusade against the Turks; he had even composed an epic poem, the Turciad, in exaltation of his idée fixe. Any prospect of a renewed crusade depended on the leadership of the militantly Catholic Habsburg powers of Austria and Spain.

Richelieu, in contrast, had no interest in a crusade. His twin objectives were to restore unity and strong monarchic governance to France after the ravages of the Wars of Religion by destroying Huguenot and aristocratic power, and to undermine Habsburg hegemony abroad. Richelieu gradually persuaded Father Joseph that the power destined to lead a new crusade was the monarchy of St Louis, epitome of the crusader. By working to make France great under a strong centralized monarchy Tremblay would help to bring the prospect of an anti-Ottoman crusade closer.

Richelieu’s claims were lent credibility by the blunt refusal of Austria and Spain to contemplate a crusade against the Turks: for them the crusade that mattered was the Counter-Reformation. Eventually Father Joseph allowed himself to be persuaded and entered into a Faustian pact with Richelieu. Arguably, the Capuchin’s greatest service to Richelieu was his tireless lobbying at court for the disgraced bishop to be allowed to return from exile in Avignon, after his career had been interrupted by the palace revolution in which Louis XIII seized power from his domineering mother.

After four years Richelieu returned, became a cardinal in 1622 and the king’s first minister in 1624, always with the faithful Father Joseph nearby. By 1628 the Richelieu/Tremblay domestic policy significantly advanced with the fall of La Rochelle and the breaking of Protestant power in France. The nobles, too, were ruthlessly subdued, and several revolts by the common people, taxed beyond endurance, were put down.

This stern reduction of the Huguenots contrasted with French foreign policy. In the Valtelline, a strategically important valley in Lombardy, French troops opened fire on Papal forces in support of the Protestant Swiss canton of Grisons. Richelieu subsidised Protestant Dutch, Danish, German and Swedish armies fighting against the Habsburgs, besides also suborning the Catholic German princes to resist Emperor Ferdinand II. It was Richelieu who persuaded Gustavus Adolphus of Sweden to invade Germany.

Father Joseph’s influence reached its apogee at the Diet of Ratisbon (Regensburg) in 1630. Attending as Richelieu’s representative, he persuaded the mainly Catholic Electors to frustrate Ferdinand II’s principal objective: election of his son as King of the Romans to ensure a Habsburg imperial succession. Having subverted the Emperor’s dynastic security, Joseph then worked on Ferdinand himself. Incredibly, he persuaded him that Gustavus Adolphus was no threat and to dismiss his formidable general Wallenstein and half of his army. The Capuchin had done more damage to Austrian power than if he had won a major battle in the field.

He was openly reproached, however, at Regensburg for disgracing his habit by prolonging war and suffering. The relentless purpose of Richelieu and Tremblay was to keep the disastrous Thirty Years War raging for as long as possible, to exhaust the Habsburg monarchy, justifying their conduct with the argument that the Protestant powers would also be badly weakened.

Moral judgements are problematic in historiography, but while the spectacle of two churchmen cynically undermining the Counter-Reformation in Europe and systematically weakening the two leading Catholic powers might raise eyebrows, there are also more fundamental considerations. The Thirty Years War (1618-1648) was the worst catastrophe, in human terms, to afflict Europe since the Black Death. In 1618 the population of Germany was 21 million; by 1648 it was reduced to 13 million, at a time of population growth elsewhere.

It was a war that grievously affected civilians. Famine made cannibalism almost commonplace and there was an outbreak of sadistic murder and torture that might have prefigured the Nazi and Communist era. The flames of this holocaust were sedulously fanned by Richelieu and Tremblay. Richelieu is easy to understand: his ruthless Realpolitik would have been appreciated by Bismarck. But Father Joseph presents a more challenging psychological conundrum.

Tremblay exceeded the normal asceticism of his Order, practising mortifications beyond those of the most zealous. Much of his day was spent in prayer, a subject on which he had written much, and on occasion he entered into spiritual rapture. Works of meditation and spirituality flowed from his pen. He was an untiring spiritual father to his protégées the Daughters of Calvary and many other seekers after God. His missionaries worked from Canada to Abyssinia; he personally converted many souls. Unlike Richelieu, he never embraced any worldly benefits or vanities.

If Père Joseph had not engaged in politics, he would have been a likely candidate for canonization; but, as it was, any Devil’s Advocate would have invoked the smoking ruins of the German states as an insuperable objection. It is certain he was not a conscious hypocrite. Despite his formidable intellect he had allowed a persuasive statesman to imbue him with the delusion that helping to build a centralized secular nation state was somehow going to restore to France the crusading mission of St Louis. In reality, such embers of the crusade which occasionally flickered to life – at Vienna in 1683 or Belgrade in 1717 – were rekindled under Habsburg auspices.

Father Joseph died in 1638, followed by Richelieu in 1642. The key question is: how far did they succeed? In the short-to-medium term, they were successful. France was restored to cohesion under a strong but not “absolute” monarchy (the first absolute ruler of France was Napoleon Bonaparte). Habsburg power never fully recovered from the check it received from France in this period. France succeeded the Austro-Spanish axis in European hegemony in the late 17th and throughout the 18th century.

Yet the long-term effects were disastrous for France. Richelieu’s obsession with natural frontiers and strong border fortresses led to expansionist policies and a mindset that culminated in the delusional folly of the Maginot Line. By replacing the old paternalist Capetian monarchy with the impersonal concept of ‘the State’, equally compatible with republican government, the foundations of the Bourbon throne were undermined. The proliferation of bureaucracy, the relentless ratchet of taxation to sustain grandiose adventures, the emasculation of the higher nobility – all this paved the way for the French Revolution.

As for Father Joseph Leclerc du Tremblay, he began his association with Richelieu on equal terms – apart from their respective clerical rank. But from the time he surrendered his personal Holy Grail – his vision of the Crusade – to embrace Richelieu’s less idealist ambitions, he was a soul diminished; a subordinate in a service that too often accorded ill with the habit that earned him his nickname of Éminence Grise. In that respect, it is possible to see his life, for all its diplomatic and spiritual brilliance, as a personal tragedy.