Oda Nobunaga (1534-1582) was considered the first to truly attempt to unify the fractured island of Japan after the collapse of the structure established by the Hōjō clan in 1467

Nobunaga was born in the Owari Province to a lesser daimyo family.  As a child he was considered peculiar because he would leave the palace and play with children in town.  This was odd because they were not Samurai and his family thought this was a sign of weakness.  His behavior earned him the name of Owari no Ōutsuke, the Fool of Owari.  Even in his youth he began to experiment with the newly arrived firearms from Kyushu.  When his father suddenly died in 1551 Nobunaga overthrew the incense and let his emotions overwhelm him.  The incident convinced the upper echelon of the Oda Clan that he was unstable and not prepared to assume control.  Nobunaga was the legitimate heir and his unwillingness to conform to the normal idea of a daimyo fractured the province.  Brother was now pitted against brother.  Nobunaga conquered and unified the Owari province then gained alliances with the Imagawa Clan of Suruga and the Kiwa Clan of Mikawa.  These alliances ensured that the northern borders of Owari would be secure and allowed Nobuhaga to track his brother down.  Defeating him and other rebels he pardoned his brother.  When Nobunaga's brother planned to rebel again his life was not spared.   By 1559 he gained legitimate control of all of Owari.

In 1560 the alliance with the Imagawa Clan collapsed as when their Daimyo Yoshimoto marched on Kyoto with an army of some 30,000.  Nobunaga used his knowledge of the area, deception, surprise, luck and the weather to defeat the force with 1,800 men.  The battle of Okehazama ended with Yoshimoto's head in Nobunaga's possession and the Imagawa Clan in ruins.  Nobunaga formed an alliance with Tokugawa Ieyasu who was able to able to assume power in the vacuum left by the battle.  

The most controversial policy Nobunaga followed that made him an outsider in his own culture was his acceptance of Christianity.  His actual acceptance of the foreign religion is debated by scholars because he used it in order to crush his enemies, most notably the Ikko-ikki monks.  These monks caused much distress in the provinces of Echizen and Kaga stirring up dissent and even rebellion.  Ikko-ikki monks were not the peaceful praticoners of Buddhism that contemporary Westerners envison, rather they were fierce warriors who carried mobile shrines into the battlefield.  Often Buddhist soldiers refused to fight them but under the banner of Christianity Nobunaga's troops did not have this problem.  Nobunaga completely destroyed their monastery at Mount Hiei, slaughtering 30,000 followers and ordering the remaining Ikko to hunted down and executed.

 Under the growing power of Nobunaga an alliance was formed to stop his grasp for total power.  The most powerful daimyo in this alliance was Takeda Shingen.  Shingen's forces defeated one of Nobunaga's armies in 1574 and almost tipped the scales of Japanese history.  However his sudden death seriously weakened the entire Takeda Clan.  Shingen had introduced the mass calvary charge to Japanese warfare and had used his superb horse and well trained men to dominate northern Japan.  To Westerners the calvary charge was integral in battle but Japanese combat had developed differently.  Originally battles had been very small scale.  Only true samurai retainers fought.  Normally they were mounted and their primary weapon was the bow.  Unlike modern understanding the sword was not the main weapon of the samurai.  The sword was their soul, it made them a samurai and only they could carry them.  Combat in the early Sengoku period was among individuals and once a samurai defeated his enemy he removed the head and returned it to his daimyo for payment.  Some samurai preferred to scour the field and bring the heads of men who they did not know back (samurai declared their name before single combat) and made the viewing of heads necessary.  Shingen's calvary charge shattered battle formations and proved to be the most innovated tactical development to that point.  Nobunaga's dance with foreigners proved to nullify that advance in less than a generation.  Nobunaga's fascination with Western firearms had advanced with his age and he was now directly importing them.  He was the first to integrate guns into his armies and developed both offensive and defensive tactics.  Shingen's son, Katsuyori marched on Tokugawa and laid siege to one of his castles in the Mikawa province in 1575.  Nobunaga and Tokugawa sent a small force to raise the siege.  Confident of his men's superiority, Takeda soldiers were considered to be the finest in Japan, Katsuyori immediately went to meet the allied force.  Nobunaga arranged 1,500 arquebuses behind a bamboo stockade in lines of three.  This allowed for the slow firing weapons to get a round off every fifteen seconds.  Katsuyori's calvary was eradicated and his spearmen fared no better.  The Takeda Clan became contained to their home province of Kai until Nobunaga completely overthrew them.

http://www.youtube.com/watch?v=hVmgT6cyE0Q

Nobunaga's other outsider policy of disregarding rank over ability would be more dangerous than protecting foreigners and embracing Christianity. In 1582 a sandal bearer peasant named Hashiba Hideyoshi, who would eventually succeed Nobunaga in power and conquer all of Japan under the name Toyotomi, invaded the Bitchu province for Nobunaga held by the Mori Clan of Southern Honshu.  Hideyoshi had been promoted to a general equivalent position and this did not sit well with Nobunaga's samurai commanders. Hideyoshi laid siege to the Mori castle in Bitchu and defeated an army led by the Mori daimyo Terumoto.  To show his respect to Nobunaga, Hideyoshi requested reinforcements.  Nobunaga sent the samurai Akechi Mitsuhide to Bitchu the retired for a small reprive at temple near Kyoto before invading the island of Shikoku.  Mitsuhide felt this task was too much of an in justice to his rank and turned his army on the lightly defended temple.  Nobunaga committed the ritual suicide of disemboweling ones self before being beheaded by a trusted second known as seppuku.

Nobunaga's legacy in Japan is great.  He is consider the first great unifier of the war marred nation.  Using the idea of tenka fubu or uniting all under heaven by the sword Nobunaga solidified his immortality in the hearts of the Japanese people.  He was the first Japanese person to be mentioned in Western histories and many of his policies isolated him from insider Japanese culture of the Sengoku period but through shear force of will, use of European technology and religion, tremendous battlefield acumen, and shrewd diplomacy he is celebrated in movies, anime, manga, video games, and festivals today.  

http://www.wsu.edu/~dee/TOKJAPAN/ODA.HTM

http://www.britannica.com/EBchecked/topic/424975/Oda-Nobunaga

http://www.biographybase.com/biography/Oda_Nobunaga.html