War
Etymology: The English word war derives from the 11th-century Old English words wyrre and werre, from Old French werre (also guerre as in modern French), in turn from the Frankish *werra, ultimately deriving from the Proto-Germanic *werzō 'mixture, confusion'. The word is related to the Old Saxon werran, Old High German werran, and the modern German verwirren, meaning 'to confuse, to perplex, to bring into confusion'.

History: The earliest evidence of prehistoric warfare is a Mesolithic cemetery in Jebel Sahaba, which has been determined to be about 13,400 years old. About forty-five percent of the skeletons there displayed signs of violent death, specifically traumatic bone lesions.
In War Before Civilization, Lawrence H. Keeley, a professor at the University of Illinois, says approximately 90–95% of known societies throughout history engaged in at least occasional warfare, and many fought constantly. Keeley describes several styles of primitive combat such as small raids, large raids, and massacres. All of these forms of warfare were used by primitive societies, a finding supported by other researchers. Keeley explains that early war raids were not well organized, as the participants did not have any formal training. Scarcity of resources meant defensive works were not a cost-effective way to protect the society against enemy raids.
William Rubinstein wrote "Pre-literate societies, even those organized in a relatively advanced way, were renowned for their studied cruelty.'" Since the rise of the state some 5,000 years ago, military activity has continued over much of the globe. The invention of gunpowder, and its eventual use in warfare, together with the acceleration of technological advances have fomented major changes to war itself.
In Western Europe, since the late 18th century, more than 150 conflicts and about 600 battles have taken place. During the 20th century, war resulted in a dramatic intensification of the pace of social changes, and was a crucial catalyst for the growth of left-wing politics.
Mao Zedong urged the socialist camp not to fear nuclear war with the United States since, even if "half of mankind died, the other half would remain while imperialism would be razed to the ground and the whole world would become socialist."

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.
The Human Security Report 2005 documented a significant decline in the number and severity of armed conflicts since the end of the Cold War in the early 1990s. However, the evidence examined in the 2008 edition of the Center for International Development and Conflict Management's "Peace and Conflict" study indicated the overall decline in conflicts had stalled.

Types of warfare: Asymmetric warfare is the methods used in conflicts between belligerents of drastically different levels of military capability or size.
Biological warfare, or germ warfare, is the use of biological infectious agents or toxins such as bacteria, viruses, and fungi against people, plants, or animals. This can be conducted through sophisticated technologies, like cluster munitions, or with rudimentary techniques like catapulting an infected corpse behind enemy lines, and can include weaponized or non-weaponized pathogens.
Chemical warfare involves the use of weaponized chemicals in combat. Poison gas as a chemical weapon was principally used during World War I, and resulted in over a million estimated casualties, including more than 100,000 civilians.
Cold warfare is an intense international rivalry without direct military conflict, but with a sustained threat of it, including high levels of military preparations, expenditures, and development, and may involve active conflicts by indirect means, such as economic warfare, political warfare, covert operations, espionage, cyberwarfare, or proxy wars.
Conventional warfare is a form of warfare between states in which nuclear, biological, chemical or radiological weapons are not used or see limited deployment.
Cyberwarfare involves the actions by a nation-state or international organization to attack and attempt to damage another nation's information systems.
Insurgency is a rebellion against authority, where irregular forces take up arms to change an existing political order. An insurgency can be fought via counterinsurgency, and may also be opposed by measures to protect the population, and by political and economic actions of various kinds aimed at undermining the insurgents' claims against the incumbent regime.
Information warfare is the application of destructive force on a large scale against information assets and systems, against the computers and networks that support the four critical infrastructures (the power grid, communications, financial, and transportation).
Nuclear warfare is warfare in which nuclear weapons are the primary, or a major, method of achieving capitulation.
Radiological warfare is any form of warfare involving deliberate radiation poisoning or contamination of an area with radiological sources.
Total war is warfare by any means possible, disregarding the laws of war, placing no limits on legitimate military targets, using weapons and tactics resulting in significant civilian casualties, or demanding a war effort requiring significant sacrifices by the friendly civilian population.
Unconventional warfare can be defined as "military and quasi-military operations other than conventional warfare" and may use covert forces or actions such as subversion, diversion, sabotage, espionage, biowarfare, sanctions, propaganda or guerrilla warfare.

Aims: Entities contemplating going to war and entities considering whether to end a war may formulate war aims as an evaluation/propaganda tool. War aims may stand as a proxy for national-military resolve.

Definition: Fried defines war aims as "the desired territorial, economic, military or other benefits expected following successful conclusion of a war".

Classification: Tangible/intangible aims:

Tangible war aims may involve (for example) the acquisition of territory (as in the German goal of Lebensraum in the first half of the 20th century) or the recognition of economic concessions (as in the Anglo-Dutch Wars).
Intangible war aims – like the accumulation of credibility or reputation – may have more tangible expression ("conquest restores prestige, annexation increases power").
Explicit/implicit aims:

Explicit war aims may involve published policy decisions.
Implicit war aims can take the form of minutes of discussion, memoranda and instructions.
Positive/negative aims:

"Positive war aims" cover tangible outcomes.
"Negative war aims" forestall or prevent undesired outcomes.
War aims can change in the course of conflict and may eventually morph into "peace conditions" – the minimal conditions under which a state may cease to wage a particular war.

Effects: 

Military and civilian casualties: Throughout the course of human history, the average number of people dying from war has fluctuated relatively little, being about 1 to 10 people dying per 100,000. However, major wars over shorter periods have resulted in much higher casualty rates, with 100–200 casualties per 100,000 over a few years. While conventional wisdom holds that casualties have increased in recent times due to technological improvements in warfare, this is not generally true. For instance, the Thirty Years' War (1618–1648) had about the same number of casualties per capita as World War I, although it was higher during World War II (WWII). That said, overall the number of casualties from war has not significantly increased in recent times. Quite to the contrary, on a global scale the time since WWII has been unusually peaceful.
Estimates for total deaths due to war vary widely. In one estimate, primitive warfare from 50,000 to 3000 BCE has been thought to have claimed 400 million±133,000 victims based on the assumption that it accounted for the 15.1% of all deaths. Other scholars find the prehistoric percentage much lower, around 2%, similar to the Neanderthals and ancestors of apes and primates. For the period 3000 BCE until 1991, estimates range from 151 million to 2 billion.

Largest wars by death toll: The deadliest war in history, in terms of the cumulative number of deaths since its start, is World War II, from 1939 to 1945, with 70–85 million deaths, followed by the Mongol conquests at up to 60 million. As concerns a belligerent's losses in proportion to its prewar population, the most destructive war in modern history may have been the Paraguayan War (see Paraguayan War casualties). In 2013 war resulted in 31,000 deaths, down from 72,000 deaths in 1990. 
War usually results in significant deterioration of infrastructure and the ecosystem, a decrease in social spending, famine, large-scale emigration from the war zone, and often the mistreatment of prisoners of war or civilians. For instance, of the nine million people who were on the territory of the Byelorussian SSR in 1941, some 1.6 million were killed by the Germans in actions away from battlefields, including about 700,000 prisoners of war, 500,000 Jews, and 320,000 people counted as partisans (the vast majority of whom were unarmed civilians). Another byproduct of some wars is the prevalence of propaganda by some or all parties in the conflict, and increased revenues by weapons manufacturers.
Three of the ten most costly wars, in terms of loss of life, have been waged in the last century. These are the two World Wars, followed by the Second Sino-Japanese War (which is sometimes considered part of World War II, or as overlapping).