A day after the ending of the Twelve Years' Truce in January 1643, he over-exerted Spain in foreign affairs and unsuccessfully attempted domestic reform.

[9] Olivares told his uncle de Zúñiga, who was to die the following year,[8] that he was now the dominant force at court;[10] he had become what is known in Spain as a valido, something more than a prime minister, the favourite and alter ego of the king.

[21] The royal favourite, who also was Sumiller de Corps and Caballerizo mayor to the King, came to power with a desire to commit the monarchy to a 'crusade of reform',[22] with his early recommendations being extremely radical.

[26] Olivares was concerned that Spain was too attached to the idea of limpieza de sangre, 'purity of blood', and worried about Castilians' disinclination for manual work.

[30] Whilst displaying huge confidence in his own capabilities and judgment,[14] he also felt considerable 'doubt and uneasiness' over his position as chief minister to the king.

[33] Whilst living a private life of "Spartan austerity" himself,[16] Olivares was skillful in using the formal and elaborate protocol of the court as a way of controlling the ambitions of Philip's enemies and rivals.

[34] Determined to attempt to improve the bureaucratic Polysynodial System of Castilian government, during the 1620s Olivares began to create juntas, smaller governmental committees, to increase the speed of decision making.

[39] Olivares wrote extensively, although there are differences of opinion amongst modern scholars on his work: some find them 'forceful, incisive and persuasive',[40] others consider them 'inflated and tortuous prose', wandering down 'interminable labyrinths'.

Under Philip III, Spain had successfully intervened in the Electoral Palatinate in combination with the forces of Ferdinand II, Holy Roman Emperor, a fellow Habsburg, during 1618–20, surrounding the Dutch provinces that had rebelled against Spanish rule some forty years before.

[46] Olivares' new influence was central to the decision to finally abandon the armistice in favour of renewed military action using the Army of Flanders and economic warfare – ending the Twelve Years Truce and attacking Dutch fleets and applying trading embargoes.

[47] This policy would ultimately fail over the next thirty years; to some the Spanish recommencement of the war has appeared 'surprising',[45] whilst it can also be explained as a misreading of internal Dutch politics.

Spain made considerable early advances against the Dutch, finally retaking the key city of Breda in 1624, albeit at huge expense.

His handling of War of the Mantuan Succession, which started to pitch France against the Habsburgs in northern Italy and would ultimately result in the French invasion of Spain,[50] has been much criticised.

[51] By 1634, France seeing the Spanish successes in Germany and the defeat of her Swedish allies, began raising the political stakes, taking provocative military action on a small scale.

[53] Nonetheless, French victory was far from certain in the 1630s;[54] Olivares' invasion plan in 1635 involved four different armies and two navies,[55] being described as 'the most ambitious military conception of early modern Europe.

[59] The destruction of the Spanish Atlantic fleet at the Battle of the Downs was another major blow, leaving a cash-strapped Spain unable to build a replacement force.

Spain in the early 17th century was a collection of possessions – the kingdoms of Castile, Aragon, Naples, Portugal, the autonomous provinces of Valencia, Catalonia and Andalusia as well as the Netherlands, Duchy of Milan etc.

This loose system had successfully resisted reform and higher taxation before, resulting in Spain having had historically, up until the 1640s at least, less than the usual number of fiscal revolts for an early modern European state.

[63] He was confident in the intellectual argument for a better defended, better ordered Spain, and never seems to have shown serious doubt that his plans would succeed,[64] or understood the growing hatred against his rule.

In 1627, Olivares attempted to deal with the problem of Philip's Genoese bankers – who had proved uncooperative in recent years – by declaring a state bankruptcy.

Faced by the Dutch capture of Brazil, Olivares turned to Portugal in 1637, attempting to raise taxes to pay for a mission to reclaim the Portuguese colony.

Catalan histories have tended to represent Olivares as deliberately provoking the rebellion of 1640, in order that he could crush it and thereby unify Spain,[68] although this is considered doubtful by most historians.

[70] Chaos ensued in the form of a major revolt; Portugal followed suit later in the year in the face of Olivares' attempts to convince its nobility to serve in the war in Catalonia,[59] with Lisbon offering Philip's throne to the House of Braganza.

[7] He retired by the King's order first to Loeches, where he published an apology under the title of El Nicandro, which was perhaps written by an agent but was undeniably inspired by the fallen minister.

El Nicandro was denounced to the Inquisition, and it is not impossible that Olivares might have ended in the prisons of the Holy Office, or on the scaffold, if he had not died beforehand of natural causes.