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To understand why and how Rocroi was considered a watershed battle, and whether it is a true indicator of the state of the Spanish military, we have to first review the evolution of the Spanish army up to Rocroi, along with the strategic and tactical situation that led to Rocroi in the first place, and then compare that with the situation of other competing militaries. To put it very simply, some think of Rocroi as the mark of a major change akin to a Civilization game whereby your 19th century Rifle Infantry units land upon AI-spawned medieval Mounted Knights unit. Rocroi was no such thing.

By the time of Rocroi, the Spanish army was considered a formidable defensive army, but not a credible offensive army.

Why was the Spanish military feared?

The fearsome Spanish tercios is already well-known, thanks to their exploits in battles from Pavia (1525) to Nördlingen(1634). The key to this superiority was not only the formation, rather there was an entire machinery of war starting from a Spanish populace that was armed and used to fighting, to military leaders, financial enablers, logistical providers, and so on.

Some refer to the School of Alba, in honor of Fernando de Toledo, the 3rd Duke of Alba, who brought a tercio of 9,000 from Italy, up the Spanish road, to the Low Countries to crush the Dutch revolt. This School of Alba is particularly important because it became the basis for the plaza de arma in Flanders. It had a recruitment system not only in Spain but also in Spanish zones of control such as in Naples, Milan, and southern Low Countries. It had a system of matriculation whereby new soldiers are placed onto garrisons in Italy or southern Low Countries, and then veterans are sent into the front lines. It established military discipline, means for advancement even to non-noble recruits, and a standard of competence. And of course, it had an evolving system of financing and leadership. The sum of all this is a seemingly inexhaustible supply of very capable military units, able to stand up and fight wherever they chose to.

Why did the Spanish military decline?

One foremost reason for the decline is best expressed by the Count-Duke Olivares, who in 1636 went on a tirade exclaiming 'the sterility of qualified men', saying donde no hay, no hay (where there are none, there are none). As the military profession gained prestige, it became sought after by nobles who lack key qualifications. The continuing outsourcing of finances led to units becoming vehicles of investment: the owner-noble sits in his veranda in Castile or Aragon while his units fight in faraway Flanders under command of a hired captain.

Notably, even the great Ambrosio Spinola, the victor of the Siege of Breda immortalized in the painting The Surrender of Breda, had to leave his post in the Low Countries to beg the court in Madrid to finally pay him for the back payment owed him. Which brings up the second major factor in the decline, namely financial insolvency. To quote the great John Elliott,

If Spinola's departure produced demoralization in the Netherlands, his arrival in Spain soon led to demoralization in Madrid.

Spinola was highly candid: the army of Flanders was in terrible shape as it was undermanned and underpaid. The theoretical numbers of men cited in Madrid were in reality 20-30% lower in the field, and even then they were left unpaid. In that era, what this meant was that soldiers and units were willing to fight only defensively, refusing to de-camp and move out unless they are paid. This is an important point which we will come to later.

The last of the great commander-financier of Spain's army was perhaps the Cardinal-Infante Ferdinand, quite possibly the most dandy military figures of the early modern era. He led a tercio up the Spanish road, beat the Swedish army in Nördlingen in 1634, and then took the post of Governor-General of the Army of Flanders seven years after Spinola left, to fill a desperate void in leadership there.

That year was important not only because of the seeming triumph of Nördlingen, but also because the courtiers of Madrid started to be aware of the reality that Castile was over-burdened and can no longer shoulder the lion's share of imperial cost. Its population had declined, and its economy in tatters, such that even Olivares conceded, I can do no more; I am all alone. Not one of Your Majesty's orders is obeyed.

How did the road turn toward Rocroi?

Even before 1634, Olivares had started to look to Aragon and Valencia, which economies was seen to be rising, and to the entire domains of the King of Spain in Iberia and the Mediterranean. He envisioned a new system whereby each province should contribute their fair share. After much deliberation and calculation, he presented the scheme of the Union of Arms in 1624, apportioning various states and provinces to raise men and pay for their maintenance such that the sum would be 140,000 men. Basically, he wanted a national army composed of contributions from each state and province. But before this plan could be put to implementation, further major challenges arose.

As the Cardinal-Infante arrived in Flanders, France changed its strategy of indirect involvement in the 30YW through financial support of Sweden's army, to outright military action both in Germany and against Spain in Iberia, starting in 1635. Thus, the Cardinal-Infante's victory led to a new belligerent -- France -- entering the fray. The French advanced into Catalonia in 1637 and in 1639 Olivares decided to undertake a counter-attack through Catalonia, at the same time pressuring Catalonia to contribute to the Union of Arms in order to defend its own territory, leading to the Catalan Revolt.

Desperate for resources, Olivares did what he could within the limits of statesmanship, namely to sell off positions of nobility, bishoprics, and military officership. Between that and increasing lack of pay, he had to significantly lower standards of discipline and justice in the army.

At this critical moment, the Cardinal-Infante died in 1641, replaced by Francisco de Melo who was a protégé of Olivares. In order to relieve pressure in Catalonia, he was ordered to mobilize the Spanish Army of Flanders to invade France from Flanders. His initial movements were highly successful, with a major victory in Honnecourt in 1642. But a year later, he lost in the Battle of Rocroi in May 1643, where the Spanish units kept their discipline for hours under fire after their allies had fled, and were allowed to leave the battlefield with their banners and weapons intact.

The Battle of Rocroi was the latest battle in a Spanish campaign to invade France from Flanders, in order to relieve pressure in Catalonia. Strategically speaking, this campaign relieved France's pressure against Catalonia. Tactically speaking, it disclosed the structural weaknesses of Spain's military.