Historians often reduce wars to a series of pitched battles, and study the tactics in them much more profoudnly than many other aspects of warfare. Somehow, we've come to view wars without a decisive battle as somehow less important, less intense - less interesting. I think it's utter rubbish.
Set-piece battles, as in, when two armies consciously deploy and face off in a [hopefully] decisive engagement are much less common in war than the average history buff seems to think, and for good reason. Giving battle is a chancey decision, as, due to the lack of control such short and very concentrated engagements impose, they tend to come down to blind luck and low-level decisions by their very nature, and can potentially result in very heavy casualties. Few intelligent commanders elect to follow such a course of action unless believing that they have a decisive advantage or simply no other way to proceed. Neither victory, nor a decisive result can ever be guaranteed. Instead, engaging in positional warfare (holding and contesting certain strategic/economic/whatever locations) and logistical/political/economic 'irregular' warfare (over a large territory in small groups - raids, counters to such, attacks on communications and logistics) have generally been seen as preferrable.
This can be seen very well in the medieval and pike&shot eras, when holding and capturing fortified positions was key, and often constant mobile warfare was conducted to facilitate such - battles generally resulting when a besieging/threatening force had to be dislodged or frustrated, or a raiding force was caught in a bad position. Even when these battles occurred, they often hardly matched the common perception of the decisive battle - only resulting in one force giving ground in order to regroup, the other unable to follow up quickly exactly because of the innumerable fortifications littering the land. Indeed, chances for a decisive victory were greater in a one sided engagement (catching one side off guard, like at Montiel or Solway Moss), rather than when two large armies had elected to fight on similar terms.
Then of course there are the numerous wars which brought profound territorial changes without ever getting the main forces of either side to commit to a decisive engagement, which is to say, pretty much ever damn medieval war.
Pitched battles - yes, they still happen. But they're hardly what determines the outcome of wars in general (an exception being short civil wars), and far from always decisive. We shouldn't judge armies purely by their effectiveness in set-piece battles, for I'd suggest we view them as the exception, rather than the rule.