Structure of the Tercios and their companies

As soon as the first Irish Tercio began to serve, the unit acquired a remarkable reputation. The brave performance of its men under fire at the siege of Rheinberg in 1606 justified the confidence invested by the high command in the soldiers. The unit was maintained during the Twelve Years’ Truce (1609–21), even though part of the Army of Flanders was disbanded. With the resumption of war, the Tercio would soon return to action in every campaign. At the end of August 1621 the Spanish forces numbered around sixty-thousand soldiers, while the Dutch army numbered around forty-eight thousand soldiers, although more than half of both forces were deployed as garrisons. This explains why the second part of the war was static. Only a small portion of the forces could be detached to fight.

During the war, the command staff of an Irish Tercio was as follows:

The decision to suppress the arquebus in favour of the musket was not made on the basis that one of the weapons was more effective than the other. Since the monthly wage of 3 escudos, given to every soldier, remained constant throughout the war, the high command wanted to reward loyalty and attract more men to serve in the army by other means; so it was decided to increase the number of musketeers per company. Every musketeer received 3 escudos de ventaja in addition to his wages, in order to help pay for match-cord and gunpowder. It also compensated for the cumbersome character of the musket in comparison to that of the arquebus. By increasing the numbers of musketeers in a company, more men were enabled to take advantage of the bonus.

In 1640 the distribution of weapons per company changed again to sixty pikemen, sixty musketeers and sixty-nine arquebusiers. The decision was taken because of a previous order of Don Fernando on 20 March 1636, which stated that the ventajas for the musketeers could only be inscribed in the books of the army after the maestre de campo had checked that the soldiers had a sufficient level of physical fitness to manage the musket.