In order to understand the Battle of Sekigahara, I should tell you about Hideyoshi Toyotomi
Hideyoshi was born to a farming family in Owari country, Aichi-gun, Nakamura (Nakamura-ku, Nagoya city) in 1537. He was born under the name of Tokichiro Kinoshita. However after being promoted in 1566, his name changed to Hideyoshi Hashiba and then finally around 1586 to Hideyosi Toyotomi.
Although, he served under Shogun Nobunaga Oda as a common foot soldier in 1554, he showed that he was a brilliant strategist and that he had superior leadership which enabled him to take up an important job in the army.
Daimyo Asai lost the fight with Oda’s forces, and his forces were destroyed, Nobunaga gave Hideyoshi north Oomi in the former territory of Asai. Hideyoshi gathered many excellent young men, such as Mitsunari Ishida, in this territory.
Nobunaga ordered Hideyoshi to march towards the Chugoku Region in 1576. In those days, the Chugoku Region was under the control of General Mori. When Hideyoshi was attacking Takamatsu Castle in 1582, he heard the news that Nobunaga killed himself in the rebellion caused by his subordinate Mitsuhide Akechi in Honnoji temple (Honnoji-Temple Incident)
Hideyoshi realized his chance to take power and made peace with General Mori in order to take revenge for Nobunaga, then returned to the Kyo with a larger army than he went. (a quick return from the Chugoku region)
Mitsuhide was astonished by the march of Hideyoshi and his lightning speed. He fought against Hideyoshi in Yamasaki, Kyo. Relizing he was over matched, Mituhide retreated only to be killed by warrior hunting (battle of Yamasaki).
Then Hideyoshi challenged Katsuie Shibata , the head of chief retainers of the Oda family who did not agree
with each other, in the battle at Shizugatake.(Shizugatake between Lake Biwa and Yogo lake)
In this battle, Hideyoshi won due to the skill of his generals such as Masanori Fukushima in the rear vanguard, Kiyomasa Kato, Yoshiaki Kato, Yasuharu Wakisaka who were trained by Hideyoshi from an early age.
After that the Chugoku and Kyushu regions were suppressed by Hideyoshi, in 1590 where he destroyed the Hojo clan of Odawara and unified Japan.
Hideyoshi who was known to have “unified the world” then worked to promote the free market, recommend commerce and industry. In addition, he placed Taiko Kenchi as head of agricultural policy, and utilized sword hunting laws which promoted a separation of farmers and samurai.
The ambition of Hideyoshi did not stop only to unify Japan, but he also wanted to conquer and rule over Korea and Mings(Chaina) as well as Europe.
In 1592 Hideyoshi sent an army of 160,000 with chief captain Hideie Ukita to Korea to make a way into Mings, and in 1597 sent an army of 160,000 under chief captain Hideaki Kobayakawa to Korea. However, his ambition was over by the time of his death.