For a noble to hire an assassin to kill another noble was a felony.
If it went undiscovered, it was one thing. But once exposed, the noble would, at best, be fined and punished. At worst, they would be stripped of their Noble Title, have their territory confiscated, and be sentenced to death.
Reisen had probably never considered that the assassination might fail. He likely never imagined that Ronin could capture the assassin alive, either.
It was this failed gambit that had backfired on him, giving Ronin the perfect excuse to take the offensive.
Therefore, the first step of his plan was to take down the Black-collar Tribe.
As for the Sain Territory, that land still belonged to the Sain Clan until Reisen was convicted. It wouldn't be appropriate for Ronin to send men to occupy it.
He first needed to prove Baron Reisen's crimes, and the upcoming year-end birthday banquet for Lurans, who was turning seventy-six, was undoubtedly the perfect stage.
