I have just started reading James Whitman’s The Verdict of Battle, which is a study of the law of war and the history of battle warfare in the eighteenth century. David Bell has reviewed the book here. I expect that I’ll have more to say about the book as I make my way through it, but for now I will just present a passage for your consideration. Here Whitman describes the change from the confined battle warfare of the eighteenth century to more expansive, less controlled forms of warfare:
Once new republican forms began to shoulder monarchy off the stage, once war cease to be a means of the acquisition of (dynastic) property and became a means of spreading new forms of government through the world [bold mine-DL], war exploded beyond the confines of the classic battlefield. Once war-making ceased to be a symbolic expression of settled sovereign legitimacy and became instead a means of contesting legitimacy, it spiraled out of control.
At the same time, ominously, the historic understanding of battle as a “tacit contract of chance” vanished. Eighteenth-century law treated battles as wagers, which meant that commanders and lawyers alike accepted the proposition that great questions of state could be decided, in some measure, by chance or Fortune. This willingness to accept decision by chance broke down dramatically in the nineteenth century. Battle ceased to be the kind of legal wager eighteenth-century Europeans had conceived it to be. Instead observers began to think of battle increasingly in millenarian terms, as a way of making history. Battle became the subject of a kind of Great Event theory of history, close in spirit to the Great Man theories that also swept European culture in the early decades of the nineteenth century. Instead of serving as a procedure to settle conflicts between dynastic monarchs, it began to represent something much grander and much more dangerous: the verdict of history [bold mine-DL]. War ceased to belong to the realm of Fortune and entered the perilous realm of Destiny. (p. 22-23)



Those regulated wars of the 18th/19th century were a fairly short time window, generally speaking.
To an extent, war was “somewhat tamed”, particularly by being monopolized by some very well defined interest groups.
In todays age, war is no longer monopolized, as such agreements on how it is to be waged will rapidly break down.
This is further reinforced by the USAs behaviour, if the strongest nation breaks the laws of war, those that deem itself threatenend by it will pay scant regard to laws themselfs.