Man has been at war since Cain killed Abel. Because man's first enemy is himself, he is constantly at war, killing others in the vain hope that he can subdue what John Knowles in his novel A Separate Peace termed "special stupidities" in himself because "wars were made instead by something ignorant in the human heart." Indeed, it is this private evil which causes man to harm others. It is an impulse toward destruction driven by greed, envy, lust, pride, wrath,--those "special stupidities" that are often called appropriately "the deadly sins," flaws in the souls of man since he lost his initial perfection bestowed upon him by his Creator. Thus, the causes of ancient wars are the same as for modern wars.
In his The Clash of Civilization, Samuel P. Huntington contends that conflicts develop among peoples "along the cultural fault lines separating these civilizations from one another." And, cultural fault lines are often formed by religious beliefs, what is in their hearts. Certainly, more people have been killed for religious reasons than for any other reason.
Author Ian Morris of The Atlantic asserts that, similarly to Darwin, one reason why war has only become common in the past 10,000 years is because war was actually used to create more peace and stronger, larger societies. Morris compares society of the 20th century to society of the Stone Age to make his claim. He starts by pointing out that, during the Stone Age, people only lived in groups of "a few dozen." If there were villages, the villages only consisted of only "a few hundred" people. The very few towns that existed only consisted of "a few thousand" people ("The Slaughter Bench of History"). However, within these small societies, a great deal of killing could take place in terms of vendettas and raids. It can be estimated that as much as 10 to 20 percent of individuals who lived during the Stone Age "died at the hands of other humans." In contrast to the Stone Age, even though the industrialized 20th century has been ravaged by two world wars, genocide, and even nuclear bombs, the fact is that all of those calamities only led to a mere 1 to 2 percent decrease in our planet's population of 2.5 billion. Hence, even though the 20th century seems more ravaged by war than ever before, the fact is that since the world's population is far greater than it was before, we are actually less effected by war, showing that the 20th and 21st centuries are actually far more peaceful than earlier centuries, as Morris claims.
Morris expands his argument further to explain exactly why in the past 10,000 years war has actually created more peace. As things progressed during the course of 10,000 years, the vendettas and raids of the Stone Age turned into wars, and the "winners of war incorporated the losers into larger societies." A result of these larger societies was that now their governors needed to find a way to strengthen these new, larger societies. Hence, governments began suppressing the violence that ran rampant in the Stone Age. Morris adds that the only reason why these governments suppressed violence is because it's far easier to govern well-behaved citizens than violent citizens, and suppression of violence led to the 90 percent reduction in violent death that was once seen in the Stone Age. Granted, governments' methods of suppressing violence was even just as violent as the violence of war, and war was even used in instances to suppress violence; regardless, suppression of violence continued to increase populations and stabilize growing society. Hence, all in all, Morris argues that war became the one method humanity found to cope with creating stronger, larger societies with increased populations, and war ultimately created more peace.
The space on enotes is not enough to even begin to effectively answer the complexity of this question. At the same time, I think that it might lie past the pay grade of anyone on the site to suggest they have the definitive answer. I attempt it, and do so with the understanding that more people can and should respond to build a dialogue about the nature of the question.
It is difficult to figure out why warfare has become common in the past 10,000 years. It's interesting to approach the question from the point of view of Modernist and Post- Modern philosophers. An effective way to analyze the reality for war throughout human existence could be to use the teachings of Marx, Darwin, Freud, and Foucault as a guide to examine why wars have been waged. Each philosopher's approach to how human consciousness is constructed suggests that as long as people have existed, wars have always been there.
Marx would assert that there has always been a materialist cause to the nature of war as long as human beings have existed. The battle over depleting resources could be seen as one reason why wars are embraced. One group of people in tribal fashion coveted a set of resources that another possessed or desire to possess. This triggered the reality of war. There has always been a desire to control what is perceived as the "means of production" or the means through which power is disseminated. Whether it is over food sources or the control of nations, Marx sees the battle for the control of material elements as critical to the cause of war. For Marx, this desire for objectified forms of power is why wars are waged. For example, one of the first wars in ancient Mesopotamia between Sumer and Elam was fought for the "spoils" that were "the weapons of Elam." In the Kurukshetra War, the desire for material control of the kingdom helps to launch armed conflict. In both of these settings, the desire to materially appropriate the world in accordance to one's own subjectivity is what drives the tendencies of war.
Darwin might argue that war is a part of the evolutionary pattern of human beings. Darwin suggested that evolution is a process that can really only be measured in geological time. The need for nature to determine what organisms survive and which do not is dependent on means through which "weaker" elements can be weeded out. Darwin saw the evolution of all organisms as predicated upon natural selection, or "those mechanisms that contribute to the selection of individuals that reproduce without regard to whether the basis of the selection is heritable." Natural selection is the way through which organisms survive through a "survival of the fittest." In Darwin's view, there are aspects of development where those deemed weaker in terms of "natural fitness" are weeded out. War is a part of this. It is the means through which the survival of the species is tested and almost part of the natural cycle of human beings. For Darwin, nature is a realm in which the struggle to survive is definitive in what it means to be a living organism. War is the embodiment of that struggle for human beings. There is an element of adaptability that is critical to Darwininian evolutionary theory. Organisms that can successfully adapt to their conditions and surroundings survive, while those who don't don't. This can be seen as a reason why war exists. It could be seen as the representation of organisms that did not survive the process of adaptability, could not accept the conditions of adaptability, or simply understand the reality of adaptability. Wars have been ways in which human beings have appropriated the natural mode of "weeding out" organisms that could not survive. The evolutionary theory of Darwin's universe is applicable to human beings who engage in war. As human beings are natural extensions of this world, perhaps it makes sense that our engagement in war is reflective of natural tendencies of existence.
Another potential reason for why war has been embraced could be due to the psychological reality of aggression. Freud would assert that war is so common to human development and advancement because it represents the release of the "Thanatos" death instinct that exists within human beings. War is an outlet, a form of release, where individuals embody the destructiveness of the aggression that is intrinsic to their nature. Freud would suggest that the reason why wars have become such a constant in human progression is because it represents a socially acceptable way to allow indulgence in the aggressive death instinct. The tribal mentality that is intrinsic to war on all levels is a reflection of the aggressiveness that Freud believed was intrinsic to human identity:
A tribe is a society tracing its origin back to a single ancestor, who may be a real person, a mythical hero, or even a god: they usually view outsiders as dangerous and conflict against them as normal. The possession of permanent territories to defend or conquer brought the need for large-scale battle in which the losing army would be destroyed, the better to secure the disputed territory. The coming of `civilization' therefore brought the need for organized bodies of shock troops.
The notion of tribal construction that Anglim, et al puts forth suggests that war is a socially sanctioned way of enabling our aggressive tendencies to emerge. Freud would point to this condition as part of the reason why war has never escaped the definition of what it means to be human.
If we wanted to take one more philosopher's approach to power, we could go Postmodern with Foucault. Writing in the wake of World War II and in the midst of the Cold War, he argued that the idea of power is the most critical element in defining human beings, their relationships to one another, and their relationships to themselves. For Foucault, power is the "diffusive" element rather than a "concentrated" one, according to Gaventa. Foucault sees power as everywhere and a reflection of everything because it is the organizing principle within human appropriation:
The strategic adversary is fascism... the fascism in us all, in our heads and in our everyday behavior, the fascism that causes us to love power, to desire the very thing that dominates and exploits us.
Foucault would see war as reflective of why wars have been so common within human construction. Wars are the ultimate statement of power. The "everyday behavior" that power drives finds its ultimate zenith in war. War is the complete realization of how much human beings love power, enough to kill another and take what is theirs as a result. In the work of Foucault, one sees that wars and human consciousness are linked to one another because of the inability to recognize and control the "strategic adversary" of power.
Modern wars can be reduced to any or all of these explanations. Modern wars have been predicated upon control of the means of production. For example, World War I could be seen as having been waged because of the desire to take the productive capacity of other nations. For example, the French colonies were coveted by the Germans, who owned colonies of their own. Wars could be seen as waged as a result of the death instinct that manifests itself in savage cruelty. The legacy of genocide that has become so associated with modern war as well as the barbarism of modern war are representative of that. The advancement of war in the modern setting has an evolutionary aspect to it, as well. Nations use "smart bombs" in the modern sense as a type of "fitness" to war conditions. The evolution of human beings has ben seen in how war has evolved, but yet never truly disappeared as a constructed means of natural selection. Finally, power is what guides the modern notion of war. To exert power over that which is seen as "rogue" or that which is seen as "the other" are conditions of such a reality.
In order to answer this properly, we need to look at the time frame referred to in the question. 10,000 years before the present is the time that is generally given as the beginning of widespread agriculture. If we accept the idea that war was not common before that, we will have to think about why agriculture would have led to war.
We can argue that agriculture led to war in at least two main ways. First, agriculture created the need for societies to have defined territories. Societies needed to have territories that were clearly their own where they could farm on a permanent basis. The competition for the best lands could have helped lead to wars. Second, agriculture made war more possible. Agriculture allowed societies to build up surpluses of food. These surpluses made it possible to have centralized governments that would instigate wars in order to enlarge their territory and their prestige. The surpluses also made it possible for there to be trained soldiers whose main profession was war. With true governments and with armies made up at least in part of trained soldiers, war became much more likely. This trend continued as agricultural societies became more technologically advanced.
Current wars are typically still fought over resources and for national pride/power. Countries fight to make sure that they (or friendly governments) will control important resources. They also fight because they want to maintain their country’s status in the world or in the region. These are the most typical causes of wars in the modern world.
The sad fact is that war (or conflict) among humans seems to have been a relative constant since the beginning of recorded history and certainly earlier than 10,000 years ago. In his book War Before Civilization, for example, Lawrence Keeley has argued convincingly that archaeological evidence suggests that almost all tribes or societies we have discovered have engaged in conflict either part or full time.
As others have noted above, the reasons for war are varied--economic, geographic, cultural--but the one constant factor is that humans engage in war for every reason under the sun. No matter how far we go back in history, war is endemic in most societies--that is, war is more common than peace. Many of the world's major historic cultures--Egyptians, Jews, Greeks, Romans, Christians --waged war almost constantly for territorial and economic expansion and, in many cases, because they believed their belief systems would be an improvement on everyone else's belief system. It is no coincidence, for example, that three of the most memorable and widely read classics from Homeric and Roman literature--The Iliad, the Aeniad, and, to a lesser degree, the Odyssey--are either about war or the effects of war.
I think it is reasonable to say that over the last 10,000-15,000 years war has not become more common but its effects have become more obvious as the "world" has grown smaller and technology has made killing on mass scale ever easier. Thousands of years ago, war and conflict were very localized--confined by relatively limited geography--but today wars can involve every continent, every country, every religion, every culture. The most compelling difference is that 10,000 years ago conflict involved rocks. Today, we have the capability of extinguishing ourselves, and that is the essential difference in conflict through history.
Charles Darwin was strongly influenced by the ideas of Thomas Malthus, who said that there will always be a struggle for existence. Darwin saw how that could influence evolution. Every living creature is forced to struggle to stay alive and reproduce on the limited space available on this little planet. That includes plants, insects, animals, birds, fish, everything. Why should man be an exception? In fact, man is the biggest and most successful predator of all. We didn't get to the top of the food chain because we are nice guys. Maybe organized warfare has originated fairly recently, but the human population was relatively small until fairly recently. Our two biggest wars have occurred just within the past hundred years. It would be nice to think that it was all over now, but the human population keeps growing at an alarming rate, and the size of the planet remains exactly the same, while in the meantime we are using up all the natural resources.
The longer time has gone on the more people have found out how to communicate with one another more readily (almost like how the Big 3 had red phones set up in their office during the Cold War for direct contact with each other). In this process morals, values, and ideals begin to clash to the point where it may offend an entire country. Imperialism, capitalism, and individualism gets involved and creates even bigger problems. And what I mean this is that you'll see over all countries and throughout time that many of the people fought in wars or battles to preserve their culture(their individuality) away from those who sought to imply rules (imperialism) over them and capitalize from the losing group.
It is only from the past 10,000 years that Man started wanting more, and that want drove people to fight for what they want. Such is the cause of wars in present day as well, such as the annexation of Ukraine-out of the want for the restoration of the Russian Empire to its former glory, from Putin's perspective.
However, in modern contexts, there are many measures put in place to prevent an all-out war from occurring like that of WWII, through diplomacy, negotiations, and the establishment of international bodies to ensure that "rules" are adhered to.