Battle of Aizu
In May 1868, the "daimyō" of Nagaoka inflicted high losses on the Imperial troops in the Battle of Hokuetsu, but his castle ultimately fell on May 19. Imperial troops continued to progress north, defeating the Shinsengumi at the Battle of Bonari Pass, which opened the way for their attack on the castle of Aizuwakamatsu in the Battle of Aizu in October 1868, thus making the position in Sendai untenable.