I'm gonna be honest, I find it a little odd that they had the SHIMAZU betray you rather than the Kobyakawa in the historical scenario. The Kobayakawa are historically the traitors, having only made their final allegiance to the Tokugawa after Iyeasu had a unit of muskets fire on his position.
>.>
<.<
For those who don't know the Battle of Sekigahara was basically the deciding point of the future shogunate. The two sides clashing were those who supported the Toyotomi's (Hideyoshi was the first man in the era to unite ALL of Japan under his banner, then launched a few unsuccesful attacks on Korea before dying and having everything splinter), and the "upstarts" under Iyeasu Tokugawa. Mitsunari Ishida led the Toyotomi Loyalists and had possibly one of the most bad ass generals under his command (Yukimura/Nobushige Sanada).
After Sekigahara Tokugawa basically mopped up the last of the Toyotomi and set up the 200 year long reign of the Tokugawa Shogunate that put Japan into a technological and sociological stasis that took Oliver Hazard-Perry opening their port with cannon to break them out of (and trigger a fairly bloody rebellion).