To our modern minds, reading medieval military history can be quite puzzling as often the kings and armies involved don't behave as we'd expect them to if they were trying their hardest to "win the war" as we'd imagine they'd want to.
A modern theory of warfare is to seek out the enemy on the battlefield, destroy their army, leave them defenseless, and then capture their territory. By contrast in Medieval warfare the armies seem to regularly miss chances to force battle on their opponents, they fail to optimise their weapons systems to get maximum advantage, they spend inordinate amounts of time pursuing things other than battle like sieges and raids, and they often seem to wander around aimlessly rather than striking strategically at enemy territory.
Some of this is due to the logistic difficulties of the time as other answers have noted, but what has been missed is that the Medieval mind clearly has a different conception of what warfare is and how it should be properly done. What explains the difference is the importance of religion - unlike today, everybody in the Medieval World believed in God.
To the Medieval mind the outcome of battle is a direct judgment from God and therefore understandably Medieval rulers were often quite circumspect about seeking battle. Also Kings (and not just oneself but the rulers of rival kingdoms) were seen as having a divine right to rule ordained by God - so even when they fought it was restrained by a respect for the others authority. 164
For example, when the English and French fought the Hundred Years War, they were not trying to "invade" or "conquer" each other as they did when they later fought the Napoleonic Wars. In Medieval times the aims and means were more subtle (at least when Christian rulers were fighting each other...)
I would argue that transferring a modern sensibility into a Medieval ruler and his army could certainly have profound effects. Without any concern for divine judgment, they would be unrestrained by Medieval courtesies and customs and could pursue strategic aims more efficiently (however, you might consider what the general population might think of this ruler and his army? If the people kept their religious sensibilities this ruler would certainly be seen as a crazed, illegitimate tyrant, perhaps even an antichrist).
As a final point I would note that not all pre-modern warfare is characterized by religious inspired restraint. For example, in the campaigns of <PERSON>, we see a classic example of a king who does deliberately seek decisive battle to invade his rival and overthrow them. Therefore it would be perfectly plausible for the ruler in your story to find his inspiration from the past rather then the future. 412