Travel by ship from Spain was not an option as it would expose him to risk of battle with the Dutch navy in the then ongoing Eighty Years' War, so in 1633, he went to Genoa, having quit his governorship of Catalonia where he had been trained.

He met with an army from Milan for a planned march through the famous Spanish Way across Lombardy, Tyrol, and Swabia, and then following the Rhine to the Netherlands.

The Swedish forces of Bernhard of Saxe-Weimar and Gustaf Horn desperately tried to prevent this merger, but were unable to catch up with Ferdinand of Hungary.

The King of Hungary tried to convince his cousin to stay and to strengthen their hold on Germany, but the Cardinal-Infante Ferdinand moved his troops almost immediately after the battle to continue to Brussels.

"France declared war on me (on me, not on my brother Philip) and I've since then had to fight against their armies and the intrigues of our brother-in-law Louis and his relentless minister Richelieu."

– Cardinal-Infante Ferdinand[4]In 1636, Ferdinand disempowered the last Protestant priests in the Spanish Netherlands, and continued his military counter-offensive by capturing Hirson, Le Catelet, and La Capelle, and securing Luxembourg using the usual mixed nationalities typical of the early modern age that included Croatian troops, and reaching as far a stronghold in France as Corbie during the Crossing of the Somme, threatening Paris.

[5][6] In the southern front Ferdinand lost the towns of La Capelle, Landrecies, and Damvillers to the French, but then he forced them to retreat south of Maubeuge.

In a letter to his brother the King of Spain shortly afterwards, Ferdinand described the battle as "the greatest victory which your Majesty's arms have achieved since the war in the Low Countries began".

The Dutch navy destroyed an important Spanish fleet in the Battle of the Downs, off the English coast, but it failed to prevent most of the army it was carrying, some 7,000 to 10,500 infantry, from landing at Dunkirk.

[11] In the south, after a failed attack on the Spanish fortress of Charlemont in Givet, the French army launched a great offensive upon Arras, the capital of the County of Artois.

Numerous rumours and lies floated about, and it was claimed that Ferdinand was planning to become an independent ruler of the Spanish Netherlands with the help of the French King, an enemy of Spain.

Making matters worse, the Spanish empire was under intense pressure militarily and financially; the Cardinal-Infante was even given conflicting orders to send troops to Spain to aid against the 1640 Portuguese uprising.

[13] He lacked friends growing up, and felt his interests were thrown aside in favor of political movements conducted by his father, Philip III, and the Duke of Lerma.

Before his death, he had an illegitimate daughter, Marie Anne de la Croix, born in Brussels in 1641 and died a nun in Madrid in 1715.

The Emperor (by now the Cardinal-Infante's old comrade in arms, Ferdinand III) favored his brother Archduke Leopold Wilhelm of Austria, a militarily unfortunate but otherwise capable ruler.

Madrid favored John of Austria the Younger, the twelve-year-old illegitimate son of Philip IV and the actress María Calderón.