The player play the main Battle of Sekigahara as a commander of the Western Army. The Eastern Army is controlled by the game system and attacks / counterattacks according to the rules. This system is partly based on the system of VPG's The Barbarossa Campaign.
It is said that the Battle of Sekigahara was won by the betrayal of Kobayakawa Hideaki and others of the Western Army. Therefore, when making a game, the elements of the politicking for the army of Kobayakawa Hideaki and others are usually included. However, this was an element that was prone to system problems, such as complicating the game system and allowing players who know the historical facts of betrayal to take measures against betrayal in advance.
Therefore, this game focused on "the salvo of guns on Mt. Matsuo by the Ieyasu main corps" that triggered Kobayakawa Hideaki to finally implement the betrayal. The game defines that Ieyasu gets impatient and starts advancing = betrayal = defeat of the Western Army, and sets the victory condition that the Western Army is defeated if the victory or defeat is not decided "until the Ieyasu main corps starts advancing".
This is a fairly high hurdle for the Western Army, but it is a victory condition unique to solitaire that you can challenge as many times as you want without having to secure an opponent. Try to challenge the if of history.