Nagato and Itachi would have lost.
Madara would have been stalled enough for the Gokage to fight Madara.
how could the 5 kage have beaten Itachi and Nagato? those two are wicked haxed. Nagato litterally soloed naruto and bee almost effortlessly, and Itachi well he is Itachi.
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Nagato's summons would keep the kage busy, Nagato would drain and kill tsunade well Itachi Tsyukonimi's Gaara into unconciousness. AY speedblitzes Nagato, only to have his lightning shield drained along iwth his chakra allowing Itachi to stab him with totsuka, ohnoki will then use particle style on itachi and the mizukage will had to that with the lava mist. Itachi will use clones to dodge, then place ohnoki in a genjutsu with the finger genjutsu. and the mizukage with be "almightly pulled into Nagato's rod and killed, and then well ohnoki is under Itachi's genjutsu, Nagato will finish him off.
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its quite simple really. Nagato showed he can react to Killer bee's speed whihc was close to Ay's, and Itachi's genjutsu and yata mirror with totsuka are no pushovers