Kabuto Yakushi first used the Impure World Reincarnation Jutsu to revive six Jinchuriki. No sooner had they returned than Nagato inserted Black Rods capable of controlling Tailed Beasts into their bodies. These rods were manifestations of the Rinnegan's Six Paths Power and could easily control Tailed Beasts and seal chakra. Once the rods were inserted, the six lost their free will, leaving Nagato in complete control. Even if Kabuto Yakushi released the Impure World Reincarnation Jutsu, the six would remain reincarnated as long as the rods stayed in place, their souls sealed inside their bodies.
After turning the six into Jinchuriki, Nagato stopped concerning himself with Kabuto Yakushi's next steps. He left Yahiko in charge of Kabuto Yakushi, then sought out Black Zetsu and asked, "Black Zetsu, have you heard anything about the Nine-Tails?"
Black Zetsu replied, "Yes, I've gathered some intelligence on the Nine-Tails. At present, Naruto Uzumaki and Fourth Hokage Minato Namikaze possess Nine-Tails chakra. Minato Namikaze is far too powerful, and he's proficient in Space-Time Ninjutsu, making him extremely troublesome to deal with. We should leave him alone for now. As for the other Nine-Tails Jinchuriki, Naruto Uzumaki, I hardly need to explain. He's even more difficult to handle than the Fourth Hokage. Not only is he proficient in the Flying Thunder God Technique, but he also has the White Eye and Rinnegan. Dealing with him would be even more troublesome. Recently, however, another person possessing Nine-Tails chakra appeared on the border of the Land of Fire."
"Who?" Nagato asked, puzzled.
Black Zetsu smiled. "The boy's name is Sora, and he's a monk from the Fire Temple. I investigated him, and he does have Nine-Tails chakra inside him. Presumably, when the Nine-Tails rampaged through Konoha over a decade ago, someone from Konoha collected some of its chakra