commander-in-chief of the expeditionary force against the eastern barbarians
Historical title used from 940 to 1335 for the military commander tasked with subduing eastern regions, often associated with the subjugation of the Emishi or other groups considered 'barbarians' by the Yamato court.
Seitō Shōgun was one of the official posts established from the Heian period through the Nanboku-chō period.
征夷大将軍 is the more famous 'barbarian-subduing generalissimo' title later held by the shoguns; 征東将軍 is an earlier, more specific title for eastern expeditions.
将軍 is a general term for a military commander; 征東将軍 is a specific historical title with a defined role and period.
Compound of 征 (sei, 'subjugate'), 東 (tō, 'east'), and 将軍 (shōgun, 'general'). The title was created for military campaigns against eastern regions, reflecting the historical view of those areas as uncivilized.