This isn't to say that war isn't brutal and inhumane in all eras, but in the wars within western Europe then there do seem to have been a lot of generally agreed upon "codes" in warfare, even absent a general law of war. There are some famous ones, like the general distaste of targeting officers, but also warfare seems to have been somewhat limited, as Frederick the Great was hardly sacking cities and deporting entire populations, as was the standard practice in contemporary southeast Asia. Given that European armies could match anybody atrocity for atrocity when fighting outside Europe, this doesn't seem to be due to any innate humanity, so why the "formalization" of warfare?
Or is this all a myth concocted in he face of twentieth century "total war"?
I wrote on this prior, and I still maintain that the absolute best source I have on this is Johann Ewald's Treatise on Partisan Warfare. To quote a piece from it:
*"The permanent military establishment proved an expensive but indispensable component of absolutist rule. In domestic policies it served as a means to bind the nobility to the state. . .In foreign policy it formed a readily available instrument for power politics. . .It's ranked and file, however, lacked a feeling of loyalty to the state or dynasty. Levied from among the lowest social classes, forced into military service. . .the standing armies of Europe. . .were held together by brutal force and blind drill. Too great of an investment to be risked lightly in battle and liable to desert at any moment, they were most efficient in a pitched battle over open ground, where superior drill, volume of fire, and sheer numbers decided the outcome of battles. Any kind of individual action was almost impossible with such. . .troops."*^1
So in some sense, yes, musket based warfare was born with an abhorrence to skirmishing based tactics that you speak of (targeting officers, etc.). So the very basis of why absolutist warfare existed along with its application (at least in Western/Central Europe) did not allow an irregular force to develop independently. This is also because "Little War" as it was commonly called required highly trained and motivated officers as it was a 'continuous war' and was a strictly tactical field. It was not an attractive field. *"Continually in action yet only supportive in function, outside the support structures and promotion criteria, unconventional in actions and dissolved at the conclusion of peace"*^2 -- noble officers did not want any part of this since it held no glory or advancement like the 'regular' army until much later.
More importantly though it required disciplined, quick thinking, self motivating men rather than rank and file blindly drilled and only operating out of fear of punishment and by the very basis of war described above, before popular participation and nationalist thought, irregular/skirmisher based warfare would be a difficult area to find recruits for that reason. This would come easily though first for the Austrians with their fiercely loyal Croats and Hungarians who were happy to, for instance, act as Grenzers (commonly referred to as 'peasant soldiers') in the military cordon against the Ottoman border which basically operated as a demilitarized zone on the Ottoman border where it was only comprised of these 'peasant soldiers' acting as a first line of defense.
From here irregular warfare would begin to get birthed in the form of 45,000 skirmishers pouring into Prussia and giving them a good ol' wholloping. Frederick was famous as for hating these types of troops, thinking them dishonorable, but using them out of necessity. As "Little War" grew, as did the lack of 'formalization'. That's not to imply however that war was some wholly noble affair prior to 'little war' growing in the 1740's-60's. The fact remains is that war throughout the late 17th and early 18th century was focused on slow paced sieges. Generals avoided decisive battle, as armies were too valuable, and replaced the decisive action with sieges (which became decisive in themselves). A byproduct of this was large amounts of desertion and armies roaming the countryside during siege 'acquiring' food and wine and...physical needs...from the local populace.
We look even further back and the 30 Years' War had absolutely zero shortage of sacking and total destruction of countryside. Brandenburg would lose 50% of its total population throughout the war alone. That to me is what strongly convinces me of Ewald's argument -- once those like Louis XIV consolidated their rule in the post-Westphalia world in absolutist or quasi-absolutist states, war became much more limited. Once absolutism, or what we ascribe as absolutist warfare, began to breakdown in the French Revolutionary Wars, as did this de facto formalization.
^1 Johann Ewald, Treatise on Partisan Warfare, 9
^2 Ibid. 11
General reading:
Koch, H.W., The Rise of Modern Warfare: 1600-1815
Duffy, Christopher, Fire and Stone: The Science of Fortress Warfare: 1660-1860
Weighley, Russel, The Age of Battles: The Quest for Decisive Warfare from Breitenfeld to Waterloo
Rothenberg, Gunther, Napoleon's Great Adversary: Archduke Charles and the Austrian Army 1792–1814
Ewald, Johann, Treatise on Partisan Warfare
Lynn, John, The Wars of Louis XIV