When the five Kages agreed to have a summit to discuss the pressing concern of Akatsuki, it was held in the Land of Iron, a neutral land which would also protect them. The Land of Iron is populated not by ninja, but by samurai, and the other ninja villages don't dare make a move on them. Mifune himself agreed to act as moderate the summit.
At the summit, Mifune proposed that the five Kages unite to face the overwhelming threat of Akatsuki, and offered to make an unbiased selection of whom it's supreme commander should be. He selected Danzo, the acting sixth Hokage, however it was revealed that Danzo was using a jutsu to manipulate Mifune's selection. But before a fight could break out, Zetsu appeared an alerted them to Sasuke's presence. Mifune's samurai did find Team Taka and engaged, although Sasuke was able to fight them off, as well as both the Raikage and Kazekage and escape to the inner chamber. Mifune took the chance to attack, but Sasuke managed to defend himself but lost the chance to pursue his target. Madara then appeared and told them his plans, and offered them to join him. They all refused, and Madara declared war.
After Madara's departure, Mifune once again proposed the ninja alliance, and said that the samurai would also to fight too. In the alliance, Mifune became the general of the fifth division.
When the Ambush squad led by Kankuro is about to be killed by Hanzo and his reluctant allies sent by Kabuto, Mifune rushes in the save them. As Mifune and Hanzo clash swords, they also clash words. Hanzo questions Mifune's samurai ways for allying with ninjas, but Mifune insists that he's fighting for peace. Hanzo says that he once tried to get the five great villages to unite to achieve peace, but it was an abysmal failure. Mifune then says that Hanzo gave up on his ideals and retreated into his seclusion, and that we won't lose his faith like Hanzo did. Hanzo tries to use ninjutsu, but Mifune stops him. Hanzo realizes from Mifune's moves that he fought him before, Mifune confirms this and says that he survived their encounter. Hanzo's salamander appeared from beneath Mifune and trapped him in it's mouth full of poison, but Mifune held his breath and cut the salamander in half. The two then charge at each other, but Mifune cuts through Hanzo's weapon and deals a devastating blow. Hanzo questions how it happened, and Mifune says it's because Hanzo has become weaker.
Mifune recounts their first encounter. When Hanzo confronted a squad of samurai, they fled in fear while Mifune remained to protect them, and even when faced with death. This impressed Hanzo, whom after defeating Mifune, decided to have a personal conversation with Mifune, talking about having faith and bringing peace. He said that he would take Mifune's life but in exchange would tell stories of Mifune as a hero, but Mifune said he'd trade his life for comrades' safety and Hanzo gave him an antidote to the poison. With that, Hanzo asked if Mifune would rather die with his faith, or live without it. After the story, Mifune asked if Hanzo failed his own test. Hanzo requested that Mifune and the others get at a safe distance and slashed himself across his abdomen, releasing a highly concentrated poison from his poison gland strong enough to subdue him so that Mifune's troops could seal him.
Mifune is a master swordsman and has perfected Iaijutsu, the art of drawing one's sword at break-neck speeds, and is even able to stop a ninja from using handseals. He, like other samurai, also specialize in chakra flow, streaming energy into their swords to enhance their range and power, and like the other samurai could presumably shape the energy into any weapon he chooses.