Kusegawa

The Battle of Kusegawa (杭瀬川の戦い) is the first known clash between the Western and Eastern Armies. Taking place only the day before Sekigahara, it is usually noted as a skirmish rather than a battle due to the low number of troops involved. This is pretty much ignored in the games.

Samurai Warriors
Sakon, seeing his chance to further isolate Tokugawa's troops, lures hot-headed generals into an ambush. He plans to surround them from all sides with the Western army. Hearing of the distress, Tadakatsu and Ina ride out to save their comrades. Either victor is pleased to know that their army's morale is saved.

Kessen
In Kessen, this is the first battle to take place and the tutorial battle for first time players. Both armies spot one another across the fog, hesitant to act for fear of an ambush. Undeterred, Ieyasu sounds the horn for battle and races at the chance to take Mitsunari's head. Each army rides with limited numbers and lead three generals into battle.

The player is free to win or lose the skirmish as the results do not alter their battle history. However, it does slightly change officers' allegiances to the either side.

Historical Information
A few days before October 21, 1600, Ishida Mitsunari and his troops were stationed at Ogaki Castle. They were evaluating their situation when Tokugawa's army arrived two days later at Mino Akasaka, a few miles away from their location. Seeing the massive army before them, many of the Western army soldiers began to quiver with fear. It is said that even Mitsunari was frightened by the sight.

One man who wasn't afraid was Shima Sakon. Wanting to restore the spirit of the men, he requested permission to lead a handful of men to test the enemy's skill. His request was wearily granted and he rode out with 500 men to the river dividing the two armies, Kusegawa. Akashi Teruzumi, who had at least 800 men, supported him by waiting in ambush. Hiding his troops in the nearby thicket, Sakon attacked the nearest general, Nakamura Kazuhide. Lured by the offense, Kazuhide and his closest allies, Arima Toyouji and Noisshiki Sukeyoshi, followed the retreating Sakon. As they pursued him, they were ambushed and fought blindly in the forest. Honda Tadakatsu, who noticed the ruckus, immediately ordered his comrades to retreat and they obeyed. Sakon and Teruzumi triumphantly returned and their army's morale was restored. Based on his experience on the field, Sakon advised to move the battle to Sekigahara.

In retrospect, most historians consider this a victory of sorts for the Western army. Though there were losses on both sides, they were minimal. The unit with the highest reported casualties was Sukeyoshi who lost 40 men in the assault. Even so, the impact of the skirmish is relatively minor in the larger scope of the war. A popular folktale says that Ieyasu casually watched the entire battle from Akasaka while eating his lunch; he didn't even bother to try to don his armor.

Today, Ogaki Castle is a museum for Sekigahara and is considered a national treasure in Japan.