A quick start review on Oda Nobunaga

Born on June 23, 1534, Nobunaga lived a bizarre early childhood that dubbed him "The Fool of Owari". His early life composed of curiosity of culture outside Japan and firearms, specifically Tanegashima firearms. His friends were people that were a mix of different social classes. At his father funeral he threw the ceremonial incense at the alter, furthering the "The Fool of Owari" view people had on him. People drew away from him and even his mentor Hirate Masahide commit seppuku.

So he changed. 

The Oda clan divided and his brother, Nobutomo Oda, seeking his defeat, Nobunaga appoached his first battle. Nobutomo was ultimately killed however in the process Nobunaga had Nobutomo younger brother and another ally to undermine Nobutomo. Later his other brother Nobuyuki seeked to also undermine Nobunaga. This also failed and with killing his brothers, he ended all opposition by 1559 in his clan. From here would take the world stage at the Battle of Okehazama. 

He rallied 3,000 troops to launch a surprise attack on the Imagawas clan of 25,000. The weather was stormy and the troops of the Imagawa were celebrating a victory, so their flank was open and their position in a narrow crossing provided a successful raid. Yoshimoto Imagawa was killed in this battle and his clan rapidly weakened. An alliance was made with Ieyasu Tokugawa, who were under Imagawa control before the battle, and Nobunaga would also meet Toyotomi Hideyoshi. These two would be he successors.