Though Nobunaga was Nobuhide's legitimate successor, the Oda clan was divided into many factions. Furthermore, the entire clan was technically under the control of Owari's shugo, Shiba Yoshimune. Thus, Oda Nobutomo, as the brother to the deceased Nobuhide and deputy of Owari Province's shugo (who used the powerless Yoshimune as his puppet), was able to challenge Nobunaga's place as Owari Provinces's new ruler. Nobutomo murdered Yoshimune when it was discovered that he supported and attempted to aid Nobunaga.
To increase his power, Nobunaga persuaded Oda Nobumitsu, a younger brother of Nobuhide, to join his side and, with Nobumitsu's help, slew Nobutomo in Kiyosu Castle, which later became Nobunaga's place of residence for over ten years. Taking advantage of the position of Shiba Yoshikane, Yoshimune's son, as the rightful shugo, Nobunaga forged an alliance with the Imagawa clan of Suruga Province and the Kira clan of Mikawa Province, as both clans had the same shugo and would have no excuse to decline. Additionally, this also ensured that the Imagawa clan would have to stop attacking Owari's borders.
Even though Nobuyuki and his supporters were still at large, Nobunaga decided to bring an army to Mino Province to aid Saitō Dōsan after Dōsan's son, Saitō Yoshitatsu, turned against him. The campaign failed, however, as Dōsan was killed and Yoshitatsu became the new master of Mino in 1556.
A few months later, Nobuyuki, with the support of Shibata Katsuie and Hayashi Hidesada, rebelled against Nobunaga. The three conspirators were defeated at the Battle of Inō, but they were pardoned after the intervention of Tsuchida Gozen, the birth mother of both Nobunaga and Nobuyuki. The next year, however, Nobuyuki again planned to rebel. When Nobunaga was informed of this by Shibata Katsuie, he faked illness to get close to Nobuyuki and assassinated him in Kiyosu Castle.
By 1559, Nobunaga had eliminated all opposition within the clan and throughout Owari Province. He continued to use Shiba Yoshikane as an excuse to make peace with other daimyo, although it was later discovered that Yoshikane had secretly corresponded with the Kira and Imagawa clans, trying to oust Nobunaga and restore the Shiba clan's place. Nobunaga eventually cast him out, making alliances created in the Shiba clan's name void.
The Oda clan's generals did not believe that they would win this impossible war. Only the night before, Shibata Katsuie had tried in vain to change Oda Nobunaga's mind about a frontal attack; he kept reminding Nobunaga of the joint army's complete lack of manpower compared to the Imagawa soldiers, who, according to rumors, numbered 40,000 men. Hayashi Sado no Kami Hidesada, the remaining advisor from Nobuhide's days, even argued for surrender without fighting, using the same reasoning as Katsuie. Upon this advice, according to the clan's chronicler, Nobunaga yelled:
"Imagawa has 40,000 men marching toward this place? I don't believe that. He 'only' has 25,000 soldiers. Yes, that is still too many. So, Sado, you want me to surrender. What if we do surrender? Will you get content with losing your life that way? Or what if we hold on like Katsuie wants me to? What if we stay here in this castle, lock it up, and wait until the Imagawas lose appetite and stop the siege and go home? We will be able to prolong our lives for 5 or 10 days, and what we cannot defend will still be undefendable. We are at the bottom of the pit, you know. And our fate is interesting. Of course the misery is too great, too. But this is how I see it: this is a chance in a lifetime. I can't afford to miss this. Do you really want to spend your entire lives praying for longevity? We were born in order to die! Whoever is with me, come to the battlefield tomorrow morning. Whoever is not, just stay wherever you are and watch me win it!"
Nobunaga was right; Yoshimoto deliberately leaked the highly exaggerated number of his soldiers out to scare the Oda clan, and the official chronicler of the Imagawas put it down as was usual in medieval battle records to exaggerate numbers.
Nobunaga's scouts reported that Yoshimoto was resting his troops at a place called Dengaku-hazama, near a little village named Okehazama. It was countryside that Nobunaga knew well. Dengaku-hazama was a narrow gorge, an ideal place for a surprise attack if the conditions were right. The scouts added that the Imagawa army were celebrating their victories with food and drink while Yoshimoto viewed the heads. So Nobunaga moved up towards Imagawa's camp, and set up a position some distance away. An array of flags and dummy troops made of straw and spare helmets gave the impression of a large host, while the real Oda army hurried round in a rapid march to get behind Yoshimoto's camp. Fortune, and the weather, favoured Nobunaga, for about mid-day the stifling heat gave way to a terrific thunderstorm. As the Imagawa samurai sheltered from the rain Nobunaga deployed his troops, and when the storm ceased they charged down upon the enemy in the gorge. So sudden was the attack that Yoshimoto thought a brawl had broken out among his men. He realized it was an attack when two samurai charged up. One aimed a spear at him, which Yoshimoto deflected with his sword, but the second swung his blade and cut off Imagawa's head.
Rapidly weakening, the Imagawa clan no longer exerted control over the Matsudaira clan. In 1561, an alliance was forged between Oda Nobunaga and Matsudaira Motoyasu (later Tokugawa Ieyasu), despite the decades-old hostility between the two clans. Tradition dates this battle as the time that Nobunaga first noticed the talents of the sandal bearer who would eventually become Toyotomi Hideyoshi.
In Mino, Saitō Yoshitatsu died suddenly of illness in 1561, and was succeeded by his son, Saitō Tatsuoki. Tatsuoki, however, was young and much less effective as a ruler and military strategist compared to his father and grandfather. Taking advantage of this situation, Nobunaga moved his base to Komaki Castle and started his campaign in Mino. By convincing Saitō retainers to abandon their incompetent and foolish master, Nobunaga weakened the Saitō clan significantly, eventually mounting a final attack in 1567. Nobunaga captured Inabayama Castle and sent Tatsuoki into exile.
After taking possession of the castle, Nobunaga changed the name of both the castle and the surrounding town to Gifu. Remains of Nobunaga's residence in Gifu can be found today in Gifu Park. Naming it after the legendary Mount Qi (岐山 Qi in Standard Mandarin) in China, on which the Zhou dynasty started, Nobunaga revealed his ambition to conquer the whole of Japan. He also started using a new personal seal that read Tenka Fubu (天下布武), which means "Spread the militarism over the whole land", or literally "... under the sky" (see all under heaven). In 1564, Nobunaga had his sister, Oichi, marry Asai Nagamasa, a daimyo in northern Ōmi Province. This would later help pave the way to Kyoto.
In 1568, Ashikaga Yoshiaki went to Gifu to ask Nobunaga to start a campaign toward Kyoto. Yoshiaki was the brother of the murdered thirteenth shogun of the Ashikaga shogunate, Yoshiteru, and wanted revenge against the killers who had already set up a puppet shogun, Ashikaga Yoshihide. Nobunaga agreed to install Yoshiaki as the new shogun and, grasping the opportunity to enter Kyoto, started his campaign. An obstacle in southern Ōmi Province, however, was the Rokkaku clan. Led by Rokkaku Yoshikata, the clan refused to recognize Yoshiaki as shogun and was ready to go to war. In response, Nobunaga launched a rapid attack, driving the Rokkaku clan out of their castles.
Within a short amount of time, Nobunaga had reached Kyoto and driven the Miyoshi clan out of the city. Yoshiaki was made the 15th shogun of the Ashikaga shogunate. Nobunaga refused the post of Kanrei and eventually began to restrict the powers of the shogun, making it clear that he intended to use him as a puppet to justify his future conquests. Yoshiaki, however, was not pleased about being a puppet and secretly corresponded with various daimyo, forging an anti-Nobunaga alliance.
The Asakura clan was particularly disdainful of the Oda clan's increasing power because, historically, the Oda clan had been subordinate to the Asakura clan. Furthermore, Asakura Yoshikage had also protected Ashikaga Yoshiaki, but had not been willing to march toward Kyoto. Thus, the Asakura clan also despised Nobunaga the most for his success.
When Nobunaga launched a campaign into the Asakura clan's domain, Asai Nagamasa, to whom Oichi was married, broke the alliance with Oda to honor the Asai-Asakura alliance which had lasted for generations. With the help of Ikko rebels, the anti-Nobunaga alliance sprang into full force, taking a heavy toll on the Oda clan. At the Battle of Anegawa, Tokugawa Ieyasu joined forces with Nobunaga and defeated the combined forces of the Asakura and Asai clans.
Nobunaga waged war even against Buddhists when they armed themselves and did not obey him. The Enryaku-ji monastery on Mt. Hiei, with its sōhei (warrior monks) of the Tendai school who aided the anti-Nobunaga group by helping Asai-Asakura alliance, was a particular thorn in Nobunaga's side, residing as it did so close to his residence in Kyoto. Nobunaga attacked Enryaku-ji and burnt it to the ground in 1571, even though it had been admired as a significant cultural symbol at the time, and killed between 3,000 and 4,000 men, women and children in the process.
Through the years, Nobunaga was able to further consolidate his position and conquer his enemies through brutality. In Nagashima, for example, Nobunaga suffered tremendous losses to the Ikko resistance who was led by anti-nobunaga alliance member Ishiyama Hongan-ji, including the death of a couple of his brothers. When Nobunaga finally surrounded the enemy complex, he set fire to it, again killing tens of thousands of non-combatants, including women and children.
One of the strongest rulers in the anti-Nobunaga alliance was Takeda Shingen, in spite of his generally peaceful relationship and a nominal alliance with the Oda clan. In 1572, at the urgings of the shogun, Shingen decided to make a drive for the capital starting with invading Tokugawa's territory. Tied down on the Western front, Nobunaga sent lackluster aid to Ieyasu, who suffered defeat at the Battle of Mikatagahara in 1573. However, after the battle, the Takeda forces retreated after Shingen died of illness in 1573. This was a relief for Nobunaga because he could now focus on Yoshiaki, who had openly declared hostility more than once, despite the imperial court's intervention. Nobunaga was able to defeat Yoshiaki's weak forces and send him into exile, bringing the Ashikaga shogunate to an end in the same year.
Also in 1573, Nobunaga successfully destroyed the Asakura and Asai clans, leading Asai Nagamasa to send Oichi back to Nobunaga and commit suicide. With Nagashima's destruction in 1574, the only threat to Nobunaga was the Takeda clan, now led by Takeda Katsuyori.
At the decisive Battle of Nagashino, the combined forces of Nobunaga and Tokugawa Ieyasu devastated the Takeda clan with the strategic use of arquebuses. Nobunaga compensated for the arquebus' slow reloading time by arranging the arquebusiers in three lines. After each line fired, it would duck and reload as the next line fired. The bullets were able to pierce the Takeda cavalry armor, causing chaos among the Takeda cavalry, who were pushed back and killed by incoming fire. From there, Nobunaga continued his expansion, sending Shibata Katsuie and Maeda Toshiie to the north and Akechi Mitsuhide to Tamba Province.
The Oda clan's siege of Ishiyama Hongan-ji in Osaka made some progress, but the Mori clan of the Chūgoku region broke the naval blockade and started sending supplies into the strongly-fortified complex by sea. As a result, in 1577, Hashiba Hideyoshi was ordered to expand west to confront the Mori clan.
However, Uesugi Kenshin, said to be the greatest general of his time since the demise of Takeda Shingen, took part in the second anti-Nobunaga alliance. Following his conquest of neighboring forces, the two sides clashed during the Battle of Tedorigawa which resulted in a decisive Uesugi victory. It was around this time that Uesugi forces began preparations to march on Kyoto.
Due to his defeat, Nobunaga's expansion in Noto, Kaga, and Etchū Province area was stagnant for a while. But Kenshin, who prepared to move his armies again after the battle, died from possibly Cerebral hemorrhage before moving them. According to later study, this preparation was not against Nobunaga but for attacking Kanto area, but anyway, after his death and confusion among his successors, Nobunaga started his campaign on this area again.
Nobunaga forced the Ishiyama Hongan-ji to surrender in 1580 and destroyed the Takeda clan in 1582. Nobunaga's administration was at its height of power and he was about to launch invasions into Echigo Province and Shikoku.
In 1582, his former sandal bearer Hashiba Hideyoshi invaded Bitchu Province, laying siege to Takamatsu Castle. However, the castle was vital to the Mori clan, and losing it would leave the Mori home domain vulnerable. Led by Mori Terumoto, reinforcements arrived outside Takamatsu Castle, and the two sides came to a standstill. Hashiba asked for reinforcements from Nobunaga.
It has often been argued that Hideyoshi had no need for reinforcements, but asked Nobunaga anyway for various reasons. Some believe that Hideyoshi, envied and hated by fellow generals for his swift rise from a lowly footman to a top general under Oda Nobunaga, wanted to give the credit for taking Takamatsu to Nobunaga so as to humble himself in front of other Oda vassals. Some also speculate that Hashiba or his retainers wanted to put Nobunaga in a vulnerable position in the front where he might be more easily assassinated. Others believe that Hashiba was the mastermind behind Akechi Mitsuhide's treachery.
In any case, Nobunaga ordered Niwa Nagahide to prepare for an invasion of Shikoku, and Akechi Mitsuhide to assist Hideyoshi. En route to Chūgoku region, Nobunaga stayed at Honnō-ji, a temple in Kyoto. Since Nobunaga would not expect an attack in the middle of his firmly-controlled territories, he was guarded by only a few dozen personal servants and bodyguards.
Nevertheless, Mitsuhide suddenly had Honnō-ji surrounded in a coup d'état, forcing Nobunaga to fight him. Nobunaga lost and was forced to commit seppuku. At the same time, Akechi forces assaulted Nijō Castle. Together with him died his young page, Mori Ranmaru, who had served him faithfully for many years and was still in his teens at the time. Ranmaru's loyalty and devotion to his lord were widely known and praised at the time.
Just eleven days after the coup at Honnō-ji, Mitsuhide was killed at the Battle of Yamazaki and his army was defeated by Hideyoshi, who eventually was made the rightful heir to Nobunaga's legacy.
Hideyoshi was brought up from a nameless peasant to be one of Nobunaga's top generals. When he became a grand minister in 1586, he created a law that the samurai caste became codified as permanent and heritable, and that non-samurai were forbidden to carry weapons, thereby ending the social mobility of Japan from which he himself had benefitted. These restrictions lasted until the dissolution of the Edo Shogunate by the Meiji revolutionaries. Hideyoshi secured his claim as the rightful successor of Nobunaga by defeating Akechi Mitsuhide within a month of Nobunaga's death.
It is important to note that the distinction between samurai and non-samurai was so obscure that during the 16th century, most male adults in any social class (even small farmers) belonged to at least one military organization of their own and served in wars before and during Hideyoshi's rule. It can be said that an "all against all" situation continued for a century. The authorized samurai families after the 17th century were those that chose to follow Nobunaga, Hideyoshi and Ieyasu. Large battles occurred during the change between regimes and a number of defeated samurai were destroyed, became ronin or were absorbed into the general populace.
Ieyasu had shared his childhood with Nobunaga as a hostage of the Oda clan. Though there were a number of battles between Ieyasu and the Oda clan, Ieyasu eventually switched sides and became one of Nobunaga's strongest allies.
Nobunaga's dominance and brilliance was not restricted to the battlefield, for he also was a keen businessman and understood the principles of microeconomics and macroeconomics. First, in order to modernize the economy from an agricultural base to a manufacture and service base, castle towns were developed as the center and basis of local economies. Roads were also made within his domain between castle towns to not only facilitate trade, but also to move armies great distances in short timespans. International trade was also expanded beyond China and the Korean peninsula, while nanban (southern barbarian) trade with Europe, the Philippines, Siam, and Indonesia was also started.
Nobunaga also instituted policies as a way to stimulate business and the overall economy through the use of a free market system. These policies abolished and prohibited monopolies and opened once closed and privileged unions, associations, and guilds, which he saw as impediments to commerce. Copies of his original proclamations can be found in Entoku-ji in the city of Gifu. He also developed tax exemptions and established laws to regulate and ease the borrowing of debt.
As Nobunaga conquered Japan and amassed a great amount of wealth, he progressively supported the arts for which he always had an interest, but which he later and gradually more importantly used as a display of his power and prestige. He built extensive gardens and castles which were themselves great works of art. Azuchi castle on the shores of Lake Biwa is said to be the greatest castle in the history of Japan, covered with gold and statues on the outside and decorated with standing screen, sliding door, wall, and ceiling paintings made by his subject Kano Eitoku on the inside. During this time, Nobunaga's subject and tea master Sen no Rikyu established the Japanese tea ceremony which Nobunaga popularized and used originally as a way to talk politics and business. The beginnings of modern kabuki were started and later fully developed in the early Edo period. Additionally, Nobunaga was very interested in European culture which was still very new to Japan. He collected pieces of Western art as well as arms and armour. He is considered to be among the first Japanese people in recorded history to wear European clothes. He also became the patron of the Jesuit missionaries in Japan and supported the establishment of the first Christian church in Kyoto in 1576, although he never converted to Christianity.
Nobunaga is remembered in Japan as one of the most brutal figures of the Sengoku period. But, in fact, his actions are not especially brutal for that time. Nobunaga was the first of three unifiers during the Sengoku period. These unifiers were (in order) Oda Nobunaga, Toyotomi Hideyoshi (also called Hashiba Hideyoshi above) and Tokugawa Ieyasu. Oda Nobunaga was well on his way to the complete conquest and unification of Japan when Akechi Mitsuhide, one of his generals, forced Nobunaga into committing suicide in Honnō-ji in Kyoto. Akechi then proceeded to declare himself master over Nobunaga's domains, but was quickly defeated by Nobunaga's general Toyotomi Hideyoshi.
Depending upon the source, Oda Nobunaga and the entire Oda clan are descendents of either the Fujiwara clan or the Taira clan (specifically, Taira no Shigemori's branch). His lineage can be directly traced to his great-great-grandfather, Oda Hisanaga, who was followed by Oda Toshisada, Oda Nobusada, Oda Nobuhide and Nobunaga himself.
One of Nobunaga's younger sisters, Oichi, gave birth to three daughters. These three nieces of Nobunaga all married important historical figures. Chacha (also known as Lady Yodo), the eldest, became the wife of Toyotomi Hideyoshi. O-Hatsu married Kyogoku Takatsugu. The youngest, O-go, married Shogun Tokugawa Hidetada (O-go's daughter Senhime married her cousin Toyotomi Hideyori, Lady Yodo's son.).
His nephew was Tsuda Nobusumi, the son of Nobuyuki. Nobusumi married into Akechi Mitsuhide's daughter, and was killed after the Incident at Honnō-ji by Nobunaga's third son, Nobutaka, who suspected him of being involved in the plot.
While most depictions in fiction show Nobunaga as villainous or demonic, a few show him in a far more positive light.