The Great Warpath ran along the Great Valley of the Appalachians (the light pink band on this map).
It's critical to recognize that the warpath was not necessarily the easiest path between two warring nations, but a traditional path used during military conflict. It was a system of war that even enemies agreed on. Traveling the warpath signaled to friend and foe alike that you were going to war. By obeying the same set of mores, this means that enemies had made an inter-cultural agreement of sorts, a Geneva convention for pre-contact America.
It should seem strange that such a tradition would persist. War results when the interests of two nations have become so estranged that at least one party abandons the sub-optimizing but non-zero-sum agreements of civilization to achieve its ends through zero-sum violence. At this stage, there is no negotiation or mutually agreed restraint to be had; adherence to rules when rules are being discarded in favor of brute self-interest would seem to make survival more difficult; and those so naive as to follow some code of conduct would swiftly perish from the Earth. Sometimes in violent conflict, both parties behave themselves (sub-optimize) in ways that obviously and immediately benefit them both; for example, trading prisoners, not shooting at medical personnel, etc. - but even those are quickly tossed out in times of desperation. So how could the idea of a traditional warpath survive?
To a modern military mind, it's bad enough that your army might have to travel through a single pass or over a single bridge, because that means your route will be predictable to your enemy, and they can attack you at your most vulnerable. But to voluntarily use a traditional route only because it's the traditional thing to do seems like sheer folly. If you're a Shawnee chief, it seems the best tactic would be to spread false rumors that your men have been sighted on the warpath, but meanwhile sneak over a ridge and attack your Cherokee foes in their predictable location on the path. The Cherokees might be scandalized by your dishonorable behavior, but who cares? They're dead, and you won.
Traditional Warfare vs. Rational Warfare
Traditional warfare may best be summarized by anthropologist Warren Morrill's comment regarding "wars" in the New Guinea highlands: "It's more like football. They won't fight in the rain because then their make-up would run." You would think that any neolithic people with such a preoccupation with cosmetics would quickly perish from the Earth; not only is this untrue, but until recently in history, traditional warfare was the norm. [Added later: another place where traditional warfare is clearly exemplified, precisely because it can be contrasted against the innovations that changed local norms to rational warfare, was early nineteenth century South Africa, i.e. Shaka Zulu - who "changed the nature of warfare in Southern Africa" which was previously "a ritualised exchange of taunts with minimal loss of life into a true method of subjugation by wholesale slaughter."]
And that is exactly the mystery. For the majority of human history and prehistory, the conduct of warfare probably resembled the puzzlingly cooperative military endeavors on Appalachian warpaths more than it did the rational war that modern armies conduct. By "rational" I mean warfare that is characterized by clear goal-setting, critical thinking to achieve those goals, and disregard of traditional mores in the pursuit of achieving them. Rational war can be, but is not necessarily, total war (involving the deliberate destruction of civilian infrastructure as military objectives). A hallmark of rational war is the adoption (even from the enemy) of innovations based on their effectiveness, and the elimination of military practices involving systems of custom, tradition and honor. It should be emphasized that as "rational" is used here, rational and moral will often be opposed - World War I was a major step forward toward rational war. More often than not rational warfare results in more death and suffering.
Rational War Comes to the Americas All On Its Own
It's important to emphasize that rational war is not an invention of evil Post-Enlightenment Europeans who dragged the world out of Edenic traditional innocence. In eastern North America, immediately to the north of the aforementioned warpath was the Iroquois Confederacy, who at least twice in the seventeenth century practiced total war and genocide in the pursuit of territorial expansion and elimination of adversaries (the Huron and the Susquehannock). It's clear that the Iroquois did not feel any need to avoid dishonor by keeping to the warpath, which is why they earned harsher-than-usual epithets from their neighbors, helpful nicknames like "black snakes" and "penis heads". And they didn't care, because they won.
Sometimes indigenous North Americans did have tricks that the European colonists didn't. When a 22-year-old Colonel Washington fought the French at Fort Necessity, he faced a combined force of French and Indians who used guerilla tactics - hiding behind trees, jumping out to take a shot, then hiding again. While this seems like common sense today, the French had just learned it from the Indians, and as a good British officer, Washington was of course shocked at their cowardice. After all, the honorable European way to fight in the eighteenth century was to stand in rows, fire, then kneel while you reloaded. The French and Indians didn't care, because they won (at least that battle). A little over two decades on, now-General Washington had learned his lesson, and fought his former commanders using the same tactics that had cost him Fort Necessity. The British considered his behavior cowardly, but you know where this is going. While this is far from an exhaustive survey, it certainly seems - and makes sense - that if all other things in battle are equal, if you cheat and throw out honor and tradition while rationally pursuing a clear goal, you win.
Rational War in Eurasia
The Aztecs had wheels on toys, but never thought to use them for actual work. Reading the military history of ancient Eurasia presents innovations that to us seem blindingly obvious but that nonetheless had to start somewhere. In the West at least, there was no such thing as the use of, and training in, standardized weapons and tactics (at least consistently) until they were introduced by Gaius Marius, Julius Caesar's uncle. Accounts of warfare in the ancient world often read more like confused gang-fights with 20,000 on each side than like battles.
Critical thinking about warfare was still surprisingly slow to get off the ground, and like the Eastern American woodland nations, Eurasians were ripe for exploitation. It's been said by military historians that the thirteenth-century Mongols behaved strikingly like a modern army set down in medieval Eurasia. Among other things their officer-equivalents were promoted based on - who knew - meritocracy, rather than family connections or whether they were popular with the local witch doctors and medicine men. There were no warpaths for the Mongols, though they did allow themselves to become sentimental when they executed royal enemies. When the armies of this same people encountered medieval Europeans, encumbered as they were with notions of Christendom and battlefield etiquette, the results were predictable. The Mongols didn't care, and not only did they win, they established the largest land empire ever to exist on Earth.
Game Theory: Traditional Warfare and the Long Term
Hulagu Khan, grandson of Jenghiz, whose army sacked Baghdad in 1258.
One criticism of game theory is that it tends to focus on "winning" isolated contests in the here and now at the expense of the long-term. In extreme cases these are called Pyrrhic victories. This is one limitation of individual reason in general - we employ simplifying abstractions that allow us to solve problems considered semantically in short-term memory. This has proved powerful for humans but results in unintended consequences over longer time periods than the individual's lifespan (and which are therefore more difficult to anticipate.). It's necessary to make simplifying assumptions to develop theories; but a simplifying assumption that a game is one-round or defined-length is anything but trivial to applications of game theory. In real life, games with a clearly defined number of "rounds" are very rare.
For many organisms including humans, most competitions for resources (money, food, territory mates) not only go on continuously for life, they involve frequent contact and even cooperation with conspecifics. Furthermore, game outcomes are not either-or. An aggressive animal always willing to fight may in fact win every fight, but will still accumulate injuries that a more docile animal might not. Always optimizing every round will not necessarily win the game.
The difference between traditional and rational warfare is the application of critical thinking. Humans are the only animal with a brain powerful enough that we can confuse ourselves by being able to solve short-term problems rationally instead of performing instinctive ("dumb") sub-optimization. War is a high-stakes instantiation of game theory, and players who follow traditional warfare rules would seem to handicap themselves by limiting their moves and making themselves more predictable; then again, they're behaving more like our pre-rational ancestors.
So if the Mongol model is so much more effective than the Shawnee model, why do we ever see warpaths, in eastern North America or anywhere else? Why retain any traditions in warfare when it's plausible that they're actively pernicious to the retainers?
To this point there are four possibilities that leap to mind explaining the persistence of traditional warfare. First is that traditional vs. rational models of warfare have no long-term outcome on the success of the culture that holds the associated values; they're just noise. Second, it's possible that this small survey is non-representative and there there is no problem, that just as game theory predicts there really is a global species-wide bias toward rational warfare, and that this small sample is masking it. Third is that it was just a matter of time, and we happen to be in a transition period toward rational warfare; that is, all those remaining traditional-warfare cultures in New Guinea will either have to adapt, or will be assimilated/destroyed by neighbors in the near future.
The most intriguing possibility is that there is actually pressure against rational warfare for reasons which aren't immediately obvious. An immediate parallel to game theory comes to mind, that it's better in Prisoner's Dilemma to forgive single defectors (but not double or worse defectors); otherwise the game turns into a string of mutual recriminations, to the benefit of neither player.
A mechanism for this is more difficult to construct but if these four hypotheses exhaust the possibilities and the first three are incorrect, than this must be it. Could it be (for example) that any culture willing to ignore inter-cultural military conduct codes inevitably implodes due to its disrespect for tradition (a kind of "rational warfare Fermi paradox")? If there actually is some pressure against rational war, it's difficult to imagine how such regulation is being imposed on its practitioners (that is, the Hurons and the Caliphs weren't in any position to make the Iroquois and Mongols start following the rules of honorable warfare). It's also tempting to draw an analogy between the distribution of the rational warfare meme and the self-limiting spread of a too-virulent pathogen, but the virulence mechanism is entirely different. In any event, this homeostasis-like pressure is likely visible over the extreme long-term, in complex ways, after countless game-theory rounds have elapsed. But even given the surprising persistence in modern times of traditional modes of warfare, in terms of casualties from warfare, far and away the predominant character of warfare on Earth today is rational, and it's unlikely that this will change. This is new in history as of the past century and if there will be long-term effects - ones which tend to eliminate the practitioners of rational war from circulation - we should, obviously, try to find out what they are.
