Short answers, pulled from the story.
War appeared in the archaeological record as early as the Mesolithic period, roughly 10,000 years ago. Evidence suggests a sharp rise in violent conflict as human groups began to compete for resources during this time.
The oldest known battlefield in Europe dates to 1250 BC. This site marks 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.
World War II claimed 70 to 85 million deaths, making it the deadliest war in history. The Mongol conquests follow with up to 60 million deaths, while the Soviet Union alone saw 27 million people die.
Political scientists use theories such as the balance of power, power transition theory, and the youth bulge theory to explain why states engage in war. Rationalist theories suggest war is the outcome of a diplomatic crisis which cannot be solved because both sides have conflicting estimates of their bargaining power.
The two main principles of jus in bello are proportionality and discrimination. Proportionality regards how much force is necessary and morally appropriate to the ends being sought, while discrimination determines who are the legitimate targets in a war.
Since 1945, civil wars have increased in absolute terms while great power wars and territorial conquests have declined in frequency. The types of warfare have expanded to include asymmetric warfare, biological warfare, chemical warfare, cold warfare, cyberwarfare, and nuclear warfare.