In 1644 the Irish Tercios had suffered major losses in soldiers and officers on both the Flemish and Spanish battlefronts. Since 1621, the Irish contingent had been fighting continuously without respite. During that time five Tercios had been raised and almost all had been disbanded, having suffered from high mortality rates and an inability to recruit sufficient replacements.

After the resumption of war against the Dutch in 1621, Tyrone’s Tercio represented 5.7 per cent of the infantry of the Army of Flanders. In 1628, the year for which we have the last surviving full muster of that Tercio in Flanders, the percentage had decreased to 2 per cent. The number in the ranks had been maintained between 1,571 and 983 men, while the total number of infantrymen in the army had doubled. The Irish presence was impossible to augment in the same proportion and the numbers gradually fell further. The situation further deteriorated after the rebellion of 1641. In 1644, there were only about 664 Irish soldiers serving in Flanders. This was an insignificant number considering the numerous obligations of Philip IV’s army in Flanders, since it was divided in two to defend the northern and southern Flemish frontiers.

The situation in Spain for both Tyrone’s and Tyrconnell’s Tercios developed in a similar way. When both units arrived in the Peninsula in 1638, they numbered 1,331 men; 939 and 392 respectively (although this last figure later increased to 503). In 1644, Tyrone’s Tercio ranked 244, while the former Tercio of Tyrconnell (at that time under Oliver FitzGerald’s command) had 121 men; the entire Irish contingent numbered only 365 men. The Irish Tercios deployed to fight against the Portuguese rebels did not have enough soldiers from the beginning of their service. Although the number of Patrick FitzGerald’s unit is not clear, after their arrival in Spain there were around three hundred men in the unit, and it was later amalgamated with Porter’s Tercio, which also numbered around three hundred men. Yet after the battle of Montijo in 1644, the Tercio was disbanded as its effectives numbered only 168. It was under strength. War’s attrition in casualties, infirmities, licensed leave as well as defections and desertions reduced the number of Irish effectives.