Westeros, unified by Aegon the Conqueror 300 years before the events of Ice and Fire, was in much the same situation as fifteenth-century England. The various claimants to the throne in the War of the Five Kings had much the same goals in mind as did the participants in the Wars of the Roses. Accordingly, their battles were fought to annihilate one another. Controlling territory was secondary to establishing legitimacy and neutralizing the opposition.
Medieval warfare was Vegetian in nature because the state was weak and power was tied to controlling land. Thus, Edward the Black Prince used chevauchée as an effective tactic when invading France in the fourteenth century. When a war becomes about control of a unified state-particularly in a civil war between rival factions-we instead find a strategy of seeking battle. A hundred years after the Black Prince, the combatants in the Wars of the Roses eagerly sought out and fought pitched battles for supreme dominance over a state. This is the sort of conflict we find in A Song of Ice and Fire.
http://deremilitari.org/2013/01/strategies-of-war-in-westeros/