The Ōnin War (応仁の乱, Ōnin no Ran^?) was a civil war that lasted 10 years (1467–1477) during the Muromachi period in Japan. A dispute between Hosokawa Katsumoto and Yamana Sōzen escalated into a nationwide war involving the Ashikaga shogunate and a number of daimyo in many regions of Japan.

The war initiated the Sengoku jidai, "the Warring States Period". This period was a long, drawn-out struggle for domination by individual daimyo, resulting in a mass power-struggle between the various houses to dominate the whole of Japan. During this long period three individuals emerged who would unite Japan under one rule; they were Oda Nobunaga, Toyotomi Hideyoshi, and Tokugawa Ieyasu.

The Ōnin conflict began as a controversy over who would become shogun after the retirement or death of Shogun Ashikaga Yoshimasa. In 1464, Yoshimasa had no heir. He persuaded his younger brother, Ashikaga Yoshimi, to abandon the life of a monk, and Yoshimi was named heir. In 1465, the unanticipated birth of a son to Yoshimasa put these plans in question. The infant, Yoshihisa, caused friction between the two brothers.

War is an organized, armed, and often a prolonged conflict that is carried on between states, nations, or other parties typified by extreme aggression, social disruption, and usually high mortality. War should be understood as an actual, intentional and widespread armed conflict between political communities, and therefore is defined as a form of political violence. The set of techniques used by a group to carry out war is known as warfare. An absence of war (and other violence) is usually called peace.

In 2003, Nobel Laureate Richard E. Smalley identified war as the sixth (of ten) biggest problems facing the society of mankind for the next fifty years. In the 1832 treatise On War, Prussian military general and theoretician Carl von Clausewitz defined war as follows: "War is thus an act of force to compel our enemy to do our will."

While some scholars see warfare as an inescapable and integral aspect of human culture, others argue that it is only inevitable under certain socio-cultural or ecological circumstances. Some scholars argue that the practice of war is not linked to any single type of political organization or society. Rather, as discussed by John Keegan in his History of Warfare, war is a universal phenomenon whose form and scope is defined by the society that wages it. Another argument suggests that since there are human societies in which warfare does not exist, humans may not be naturally disposed for warfare, which emerges under particular circumstances. The ever changing technologies and potentials of war extend along a historical continuum. At the one end lies the endemic warfare of the Paleolithic^{[citation needed]} with its stones and clubs, and the naturally limited loss of life associated with the use of such weapons. Found at the other end of this continuum is nuclear warfare, along with the recently developed possible outcome of its use, namely the potential risk of the complete extinction of the human species.

Takeda Shingen (武田 信玄^?, December 1, 1521 – May 13, 1573), of Kai Province, was a preeminent daimyo in feudal Japan with exceptional military prestige in the late stage of the Sengoku period.

Shingen was called "Tarō" (a commonly used pet name for the eldest son of a Japanese family) or "Katsuchiyo" (勝千代) during his childhood. When he celebrated his coming of age, he was given a formal name of "Harunobu" (晴信), which included a character from the name of Ashikaga Yoshiharu (足利義晴), the 12th Ashikaga Shogun. It was a common practice in feudal Japan for a higher-ranked warrior to bestow a character from his own name to his inferiors as a symbol of recognition. From the local warlord's perspective, it was glorious to receive a character from the shogunate, although the authority of the latter had greatly degenerated in the mid-16th century.

Both the Ashikaga and the Takeda clans descended from the noble Minamoto (源) clan. Technically, Harunobu, as well as his forefathers, had borne the surname of Minamoto. Therefore, Harunobu would be referred to as "Minamoto-no Harunobu" (源晴信) in official records kept by the Imperial Court when he was conferred the official title of "Daizen Dayu" (大膳大夫). The Imperial Court had maintained a system of ritsuryō (律令) that was parallel to the shogunate apparatus.

Oda Nobunaga (織田 信長,  Oda Nobunaga (help·info)^?, June 23, 1534 – June 21, 1582) was the initiator of the unification of Japan under the shogunate in the late 16th century, which ruled Japan until the Meiji Restoration in 1868. He was also a major daimyo during the Sengoku period of Japanese history. His opus was continued, completed and finalized by his successors Toyotomi Hideyoshi and Tokugawa Ieyasu. He was the second son of Oda Nobuhide, a deputy shugo (military governor) with land holdings in Owari Province. Nobunaga lived a life of continuous military conquest, eventually conquering a third of Japan before his death in 1582. His successor, Toyotomi Hideyoshi, a loyal Oda supporter, would become the first man to unify all of Japan, and was thus the first ruler of all Japan since the Ōnin War.

Oda Nobunaga was born on June 23, 1534, and was given the childhood name of Kippōshi (吉法師^?). He was the second son of Oda Nobuhide. Through his childhood and early teenage years, he was well known for his bizarre behavior and received the name of Owari no Ōutsuke (尾張の大うつけ^?, The Fool of Owari). With the introduction of firearms into Japan, though, he became known for his fondness of Tanegashima firearms. He was also known to run around with other youths from the area, without any regard to his own rank in society. He is said to be born in Nagoya Castle, although this is subject to debate. It is however certain that he was born in the Owari domain. In 1574 Nobunaga accepted the title of Kuge (or Court Noble), then in 1577 he was given the title of Udaijin (or Minister of the Right), the third highest position in the Imperial court.