For as long as Izuna could remember he had always had one opponent when his own clan met with the Senju in battle and, although he would never admit to it out loud, his reasons for facing Tobirama were similar to the reasons that his brother faced off against Hashirama each time. Should the two evenly matched pairs ever split apart and find others to fight there would be carnage on both sides. There were few who could hope to match any of the titans from either the Uchiha or Senju ranks.

Which is why it had been so strange for Izuna to scan the battlefield before him and realize that Tobirama was nowhere to be found; strange, but not truly a bad thing. If the Senju wished to leave themselves open to slaughter then he was more than willing to oblige them.