The Early Tudor Army
The Early Tudor Army was still very much a medieval one. The system of Livery and Maintenance, where the great landed magnates like the Nevilles and the Percys had supported private armies of their own, and which had helped fuel the Wars of the Roses, had been replaced by a system of indentured levy, where the nobles were only permitted to raise troops from among their own tenantry. The Tudor monarchs no longer permitted permanent armed retinues of mercenaries.
In time of war, Commissions of Array were issued to a lord by the King's agents. Bands of soldiers mustered under the lord or, if serving in an incorporated town's trained band, under an appointed civic officer. The unit would then march to an arranged meeting place, usually a city or large town.
Upon arrival, the lord would retain command of this mustering, but they would be grouped into a division and placed under the command of a superior officer. These divisions were then massed into three large groups, known as the Forward Battle, the Rearward Battle and the Main Battle.
Supplementing the system of indentured levy was the town and county militia. In this case, the local militia would be called to fill out the numbers of the Royal army. The militia system wasn't designed to allow a monarch to project military strength beyond the borders of his own realm, but this did not keep Henry VIII from deploying 1500 militia to defend the Calais Pale.