The Battle of Sekigahara was a pivotal battle in October of 1600 that truly unified Japan and also established a new shogunate that lasted for 268 years. The battle was fought between two factions: the Eastern Army, under the command of Ieyasu Tokugawa, and the Western Army, commanded by Mitsunari Ishida. The forces at the Battle were roughly evenly matched, the outcome could have easily been different if not for the actions of certain commanders.
"The Battle of Sekigahara,"
The Histories: Vol. 3
, Article 10.
Available at: http://digitalcommons.lasalle.edu/the_histories/vol3/iss1/10