Ishida, in his home Sawayama Castle, met with Ōtani Yoshitsugu, Mashita Nagamori, and Ankokuji Ekei.
Here, they forged their alliance, and invited Mōri Terumoto to be its head.
 They formed what came to be referred to as the Western Army.
 Mōri seized Osaka Castle for their base of operations, since most of Tokugawa's forces had vacated the area to attack Uesugi.

Ishida wanted to reinforce Mōri at the impregnable Osaka Castle.
 This would let Ishida control the capital of Kyoto and challenge the Tokugawa.
 To this end, Ishida's forces headed for Gifu Castle in order to use it as a staging area to move on Kyoto, since it was controlled by his ally Oda Hidenobu.

Back in Edo, Tokugawa Ieyasu received news of the situation in the Kansai region and decided to deploy his forces.
 Ieyasu himself commanded 30,000 men and his subordinates led another 40,000 men.
 This made up the bulk of what would later be called the Eastern Army.
He had some former Toyotomi daimyō engage with the Western Army, while he split his troops and marched west on the Tōkaidō towards Osaka.

Since the Tokugawa army departed from Edo, it could only take two roads, both of which converged on Gifu Castle.
 Ieyasu marched on Gifu while Ishida Mitsunari was delayed at Fushimi Castle.
This fortress was a halfway point between Osaka and Kyoto and was controlled by the Tokugawa ally Torii Mototada.
Ishida could not risk leaving a force that could attack his rear, so he marched on it.
It took him ten days to capture Fushimi, and in that time Gifu Castle had fallen.
 This forced Ishida Mitsunari to retreat southward in the rain.
 The rain was relevant in that the bulk of both armies were equipped with matchlock rifles (tanegashima), which required dry gunpowder to fire.

Tired from a day's march and their gunpowder wet from the rain, Ishida and his forces stopped at Sekigahara.
 Ishida deployed his troops in a strong defensive position, flanked by two streams with high ground on the opposite banks.
 His right flank was reinforced by daimyō Kobayakawa Hideaki on Mount Matsuo.

On October 20, 1600, Ieyasu learned that Ishida Mitsunari had deployed his troops at Sekigahara in a defensive position.
 They had been following the Western Army, and benefited from considerably better weather.
At dawn of the next day, the Tokugawa advance guard stumbled into Ishida's army.
 Neither side saw each other due to the dense fog caused by the earlier rain.
 Both sides panicked and withdrew, but this resulted in both sides being aware of their adversary's presence.

Ishida held his current defensive position and Ieyasu deployed his own forces.
He sent his allies' forces in a line to the front, and held his own troops in reserve.
 Around 8:00 am, wind blew away the fog, and both sides noticed their respective adversary's positions.
 Last-minute orders were issued and the battle began.
Ishida, in his home Sawayama Castle, met with Ōtani Yoshitsugu, Mashita Nagamori, and Ankokuji Ekei. Here, they forged their alliance, and invited Mōri Terumoto to be its head.  They formed what came to be referred to as the Western Army.  Mōri seized Osaka Castle for their base of operations, since most of Tokugawa's forces had vacated the area to attack Uesugi.
Ishida wanted to reinforce Mōri at the impregnable Osaka Castle.  This would let Ishida control the capital of Kyoto and challenge the Tokugawa.  To this end, Ishida's forces headed for Gifu Castle in order to use it as a staging area to move on Kyoto, since it was controlled by his ally Oda Hidenobu.
Back in Edo, Tokugawa Ieyasu received news of the situation in the Kansai region and decided to deploy his forces.  Ieyasu himself commanded 30,000 men and his subordinates led another 40,000 men.  This made up the bulk of what would later be called the Eastern Army. He had some former Toyotomi daimyō engage with the Western Army, while he split his troops and marched west on the Tōkaidō towards Osaka.
Since the Tokugawa army departed from Edo, it could only take two roads, both of which converged on Gifu Castle.  Ieyasu marched on Gifu while Ishida Mitsunari was delayed at Fushimi Castle. This fortress was a halfway point between Osaka and Kyoto and was controlled by the Tokugawa ally Torii Mototada. Ishida could not risk leaving a force that could attack his rear, so he marched on it. It took him ten days to capture Fushimi, and in that time Gifu Castle had fallen.  This forced Ishida Mitsunari to retreat southward in the rain.  The rain was relevant in that the bulk of both armies were equipped with matchlock rifles (tanegashima), which required dry gunpowder to fire.
Tired from a day's march and their gunpowder wet from the rain, Ishida and his forces stopped at Sekigahara.  Ishida deployed his troops in a strong defensive position, flanked by two streams with high ground on the opposite banks.  His right flank was reinforced by daimyō Kobayakawa Hideaki on Mount Matsuo.
On October 20, 1600, Ieyasu learned that Ishida Mitsunari had deployed his troops at Sekigahara in a defensive position.  They had been following the Western Army, and benefited from considerably better weather. At dawn of the next day, the Tokugawa advance guard stumbled into Ishida's army.  Neither side saw each other due to the dense fog caused by the earlier rain.  Both sides panicked and withdrew, but this resulted in both sides being aware of their adversary's presence.
Ishida held his current defensive position and Ieyasu deployed his own forces. He sent his allies' forces in a line to the front, and held his own troops in reserve.  Around 8:00 am, wind blew away the fog, and both sides noticed their respective adversary's positions.  Last-minute orders were issued and the battle began.
Battle of Sekigahara