War is not a modern invention but a shadow that has stretched across human history, appearing in the archaeological record as early as the Mesolithic period, roughly 10,000 years ago. While some anthropologists argue that early Paleolithic societies were fundamentally egalitarian and rarely engaged in organized violence, the evidence suggests a sharp rise in violent conflict as human groups began to compete for resources. Lawrence H. Keeley, a professor at the University of Illinois, posits that approximately 90 to 95 percent of known societies throughout history have engaged in at least occasional warfare, with many fighting constantly. These early conflicts were not the grand battles of later centuries but small raids, large raids, and massacres conducted by participants without formal training. Scarcity of resources meant that defensive works were often not a cost-effective way to protect a society, leading to a cycle of raiding that began in the richest environments. The oldest known battlefield in Europe dates to 1250 BC, marking the Bronze Age as a key period in the intensification of warfare with the emergence of dedicated warriors and the development of metal weapons like swords. This era laid the groundwork for the state formation theories of Charles Tilly, who argued that war made the state, and the state made war, creating a cycle that has persisted for millennia.
The Cost of Conflict
The human toll of war extends far beyond the battlefield, claiming lives through disease, famine, and the systematic destruction of civilian infrastructure. Estimates for total deaths due to war vary wildly, with one calculation suggesting that primitive warfare from 50,000 to 3000 BCE claimed 400 million victims, accounting for 15.1 percent of all deaths in that period. Ian Morris offers a higher estimate, suggesting the rate could be as high as 20 percent, while other scholars find the prehistoric percentage much lower, around 2 percent, similar to Neanderthals and ancestors of apes. For the period from 3000 BCE until 1991, estimates range from 151 million to several billion, with the lowest estimate calculated by William Eckhardt summing recorded casualties and multiplying their average by the number of recorded battles. The deadliest war in history, World War II, claimed 70 to 85 million deaths, followed by the Mongol conquests with up to 60 million. In the Soviet Union alone, 27 million people died, and of the 450,000 soldiers who crossed the Neman on the 25th of June 1812, less than 40,000 returned from Napoleon's retreat from Moscow, with more French military personnel dying of typhus than were killed by the Russians. War also results in significant deterioration of infrastructure, leading to famine and disease, with a medium-sized conflict reducing civilian life expectancy by one year and increasing infant mortality by 10 percent.
The motivations behind war are as complex as the conflicts themselves, ranging from psychoanalytic theories of mass rage to evolutionary instincts for territoriality and sexual competition. Dutch psychoanalyst Joost Meerloo held that war is often a mass discharge of accumulated internal rage, where the inner fears of mankind are discharged in mass destruction. Other psychoanalysts such as E.F.M. Durban and John Bowlby argued that human beings are inherently violent, with aggressiveness fueled by displacement and projection where a person transfers grievances into bias and hatred against other races, religions, nations, or ideologies. The Italian psychoanalyst Franco Fornari thought war and violence develop out of our love need, the wish to preserve and defend the sacred object to which we are attached, namely our early mother and our fusion with her. Despite these theories, few wars have originated from a desire for war among the general populace; far more often, the general population has been reluctantly drawn into war by its rulers. Maurice Walsh argues that wars occur when leaders with a psychologically abnormal disregard for human life are placed into power, such as Napoleon and Hitler, who most often come to power in times of crisis when the populace opts for a decisive leader. Steven Pinker argues that war-related behaviors may have been naturally selected in the ancestral environment due to the benefits of victory, causing humans to develop instincts for revenge as well as for protecting a group's reputation.
The Economics of War
Political scientists have developed numerous theories to explain why states engage in war, ranging from the balance of power to the security dilemma and the youth bulge. The balance of power theory suggests that states have the goal of preventing a single state from becoming a hegemon, and war is the result of the would-be hegemon's persistent attempts at power acquisition. Power transition theory posits that wars become more probable when power is more equally distributed, as a declining hegemon is challenged by another rising power. Demographic theories, such as the youth bulge theory proposed by Heinsohn, suggest that a youth bulge occurs when 30 to 40 percent of the males of a nation belong to the fighting age cohorts from 15 to 29 years of age, leading to rebellion and revolution waves. The 1994 Rwandan genocide has been analyzed as following a massive youth bulge, and the effect of economic depression upon the largest German youth cohorts ever has been used to explain the rise of Nazism in Germany in the 1930s. Rationalist theories, such as those examined by James Fearon, suggest that war is the outcome of a diplomatic crisis which cannot be solved because both sides have conflicting estimates of their bargaining power, or that states fail to make credible commitments. These theories operate under the assumption that states or international actors are rational, seek the best possible outcomes for themselves, and desire to avoid the costs of war, yet war nonetheless
Theories of Cause
reoccurs.
The morality of war has been the subject of debate for thousands of years, with the two principal aspects of ethics in war being jus ad bellum and jus in bello. Jus ad bellum dictates which unfriendly acts and circumstances justify a proper authority in declaring war on another nation, requiring that any just war must be declared by a lawful authority, be a just and righteous cause, have rightful intentions, have a reasonable chance of success, be a last resort, and have ends being sought that are proportional to means being used. Jus in bello is the set of ethical rules when conducting war, with the two main principles being proportionality and discrimination. Proportionality regards how much force is necessary and morally appropriate to the ends being sought and the injustice suffered, while the principle of discrimination determines who are the legitimate targets in a war, specifically making a separation between combatants, who it is permissible to kill, and non-combatants, who it is not. The just war theory was foundational in the creation of the United Nations and in international law's regulations on legitimate war. Religious groups have long formally opposed or sought to limit war, with the Second Vatican Council document Gaudium et Spes stating that any act of war aimed indiscriminately at the destruction of entire cities or extensive areas along with their population is a crime against God and man himself. Anti-war movements have existed for every major war in the
The Ethics of War
20th century, including World War I, World War II, and the Vietnam War, with worldwide anti-war movements occurring in the 21st century in response to the United States invasion of Afghanistan and Iraq.
Since 1945, the nature of war has shifted dramatically, with great power wars, territorial conquests, and war declarations declining in frequency, yet war in general has not necessarily declined. Civil wars have increased in absolute terms since 1945, and a distinctive feature of war since 1945 is that combat has largely been a matter of civil wars and insurgencies. The major exceptions were the Korean War, the Indo-Pakistani War of 1971, the Iran-Iraq War, the Gulf War, the Eritrean-Ethiopian War, and the Russo-Ukrainian War. Warfare has been increasingly regulated by international humanitarian law, and battle deaths and casualties have declined, in part due to advances in military medicine and despite advances in weapons. In Western Europe, since the late 18th century, more than 150 conflicts and about 600 battles have taken place, but no battle has taken place since 1945. The types of warfare have expanded to include asymmetric warfare, biological warfare, chemical warfare, cold warfare, cyberwarfare, and nuclear warfare. Chemical warfare, for instance, was principally used during World War I, resulting in over a million estimated casualties, including more than 100,000 civilians. Cyberwarfare involves the actions by a nation-state or international organization to attack and attempt to damage another nation's information systems, while information warfare applies destructive force on a large scale against information assets and systems, including the power grid, communications, financial, and transportation infrastructures.
The future of war remains uncertain, with theories suggesting that war is a historical invention associated with certain
The Modern Battlefield
types of human societies, while others argue it is a universal human occurrence. Ashley Montagu strongly denied universalistic instinctual arguments, arguing that social factors and childhood socialization are important in determining the nature and presence of warfare. Bobbi S. Low has observed a correlation between warfare and education, noting that societies where warfare is commonplace encourage their children to be more aggressive. The statistical analysis of war was pioneered by Lewis Fry Richardson following World War I, and more recent databases of wars and armed conflict have been assembled by the Correlates of War Project, Peter Brecke, and the Uppsala Conflict Data Program. The durability of ceasefire agreements is affected by several factors, such as demilitarized zones, withdrawal of troops, and third-party guarantees and monitoring. Ceasefires may be intended to meet short-term limited needs, such as providing humanitarian aid, or to manage a conflict to make it less devastating. The immediate goal of a ceasefire is to stop violence, but the underlying purposes of ceasefires vary, and an actor may not always intend for a ceasefire to advance the peaceful resolution of a conflict. As technology advances, the methods of war evolve, from the use of poison gas in World War I to the sophisticated technologies of cluster munitions and cyberwarfare in the 21st century, challenging the very definition of what constitutes a legitimate military target and