In order to understand the Battle of Sekigahara, I should tell you about Hideyoshi Toyotomi first.
Hideyoshi was born to a farming family in Owari county, 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.