The Spanish army was by the late 1620s no longer as dominant on the battlefield as it once had been.
The feared tercio regiments, composed of well-disciplined pikemen, are increasingly appearing inflexible and outmoded in the face of the new Swedish and Dutch formations with a higher proportion of musketeers.
Philip and Olivares have attempted to address the perceived weaknesses of the army, which they have concluded is primarily due to the falta de cabezas, or a lack of leadership.
In keeping with their wider agenda of renewing the concepts of duty, service and aristocratic tradition, the king has agreed to efforts to introduce more grandees into the higher ranks of the military, working hard to overcome the reluctance of many to take up field appointments in the Netherlands and elsewhere.
The results are not entirely as hoped.
The grandees dragooned into service in this way are disinclined to spend years learning the normal professional military skill set.
By the 1630s, the king is waiving the usual rules to enable promotion to higher ranks on a shorter timescale, and having to pay significant inflated salaries to get grandees to take up even these appointments.
Philip is also notable for his interest in the Spanish armada, or navy.
Shortly after taking power in 1621, he had begun to increase the size of his fleets, rapidly doubling the size of the naval budget from the start of his reign, then tripling it; Philip is credited with a 'sensible, pragmatic approach' to provisioning and controlling it.
He is prepared to involve himself in considerable details of naval policy -- he was commenting on the detail of provisions for the armada in 1630, for example.