Sekishu is a province located at the east of Utakata, introduced during the events of Revenge of Zen. It's the homeland of the Fuka Ninja and currently ruled by the Hojo Clan. It's explained that Zen and Shu used to serve as houseguards of the Hojo Clan, and were close friends with Princess Sakuya.
Some time before the events of the game there was a war in Sekishu (possibly against the Nagao clan) that resulted in Shizuku's death. By the beginning of the story the conflict with Iwafune still plagues Sekishu, but not enough to stop the Hojo Clan from aiding the Kazama Clan in their uprising and conquest of Fudo.
The Hojo Clan have declared hostility towards the House of Ichijo, while both the Kazama Clan and Amurita Faith intend to gain Hojo's support. It's known that Hojo Ujiyasu considers Nobuteru Ichijo a pampered, weak ruler, but his opinion on Kazama and Kihan is more ambiguous, since it's only told from the perspective of a manipulative Shu.
It seems the only thing stopping the Hojo Clan from invading Utakata is their own strife against Iwafune. Even if Ichijo or Amurita claim control over Utakata, the Hojo Clan may be dissuaded from their hostile stance by the safe return of Princess Sakuya.