The battle of Sekigahara, fought in 1600 at a crossroads in Japan, unified that nation under the Tokugawa family for more than 250 years.
Sekigahara allows you to re-contest that war as Ishida Mitsunari, defender of a child heir, or Tokugawa Ieyasu, Japan's most powerful daimyo (feudal lord).
The campaign lasted only 7 weeks, during which each side improvised an army and a strategy with what forces their allies could provide. Each leader harbored deep doubts as to the loyalty of his units - for good reason. Several daimyo refused to fight; some even turned sides in the midst of battle.