"Kessen (決戦, Kessen) is a PlayStation 2 launch title produced by Koei and published by Electronic Arts. This was the first DVD-ROM format game for the PS2. It was initially the only real-time wargame game available for the PlayStation 2.
It is a real-time tactics game set in feudal Japan, focusing upon a conflict between the eastern Tokugawa clan and western Toyotomi clan (with Ishida Mitsunari as the Toyotomi's guardian and protector) shogunates at the end of Sengoku Jidai. Tokugawa Ieyasu served as the primary representative for the Eastern forces where the Western army changed leaders based on the results of select key battles.
Although it was, for the most part, historically accurate, the game contained a number of "What-if" scenarios, for example, if the Western forces are victorious at the Battle of Sekigahara (historically an Eastern victory).
Two sequels were later created, Kessen II and Kessen III. These two games introduced magical and meteorological elements to the games."
Source: Wikipedia, "Kessen", available under the CC-BY-SA License.