Short answers, pulled from the story.
War is an armed conflict between the armed forces of states, between governmental forces and organized armed groups, or between such organized groups. To qualify, the groups must operate under a command structure and have the capacity to sustain military operations. It is generally characterized by widespread violence, destruction, and mortality in pursuit of political, economic, or territorial objectives.
World War II is the deadliest war in history by cumulative number of deaths, with 70 to 85 million deaths from 1939 to 1945. It is followed by the Mongol conquests, with up to 60 million deaths. World War II was also the most financially costly conflict, with belligerents spending about a trillion U.S. dollars adjusted to 1940 prices.
The English word war descends from the 11th-century Old English words wyrre and werre, from Old French werre, in turn from the Frankish werra, and ultimately from the Proto-Germanic werzo. It is related to the modern German verwirren, meaning to confuse.
Since 1945, great power wars, territorial conquests, and war declarations have declined in frequency, and battle deaths have fallen partly due to advances in military medicine. Civil wars and insurgencies have grown in absolute terms and now make up most combat. In Western Europe, no battle has taken place since 1945.
Types of warfare include asymmetric, biological, chemical, cold, conventional, cyber, nuclear, radiological, total, and unconventional warfare, along with insurgency and information warfare. Total war discards the laws of war and places no limits on legitimate targets. Chemical warfare using poison gas during World War I caused over a million estimated casualties.
Just war theory has two principal aspects: jus ad bellum, the right to war, and jus in bello, the right in war. Jus ad bellum sets six criteria for declaring a just war, including lawful authority, just cause, and war as a last resort. Jus in bello turns on proportionality and discrimination between combatants and non-combatants.
Theories of war include psychoanalytic, evolutionary, economic, Marxist, demographic, and rationalist explanations, with no consensus on which are most common. James Fearon's rationalist account names issue indivisibilities, incentives to misrepresent, and commitment problems. Marxist theorists like Lenin and Rosa Luxemburg held that modern wars stem from capitalist competition for markets and resources.