Stupid logic. Akastuki gather 7 bijuus altogether, and each bijuu had the power to destroy a single village. There's only five great nations in existence. During the war the edo tenseis where used against the any in some cases and if not for Itachi, this whole plan would've gone down smoothly. Edo Madara only fought the kaguya, while all the other edo tenseis where released way too early.
Akatsuki that is alive and capable enough to destroy a village with two members is more than enough.
That would be the equivalence of what Japan did in Pearl Harbor or Germany during the blitzkrieg; make fast and devastating attacks in succession, thinking that it would overwhelm your enemies, except in both cases it failed because both Germany and Japan severely underestimate the amount of opposition and the cohesion of said opposition. That sort of plan would work at first the same way the blitzkrieg worked with astounding success at first, but once every ****ing ninja on the planet, along with all the samurai, and other fighting personnel, the moment the synchronicity and sheer immensity of logistics, agencies and more combined the Akatsuki won't be making it out alive.
Not to mention Orochimaru, who was an enemy of the Akatsuki, would have most likely teamed up with the 5 nations against Akatsuki.
Have you also forgotten that Itachi was secretly working with Konoha, which Obito knew and admitted that with Itachi alive such a brazen invasion would never work? How about the fact that when Itachi actually did die most of the Akatsuki were dead anyways?
You severely underestimate the 5 Kage, who can easily take on at least half the Akatsuki. Tsunade gets a bad rep for her performance against Pain, even though she spent all of her chakra healing the leaf village. Against Madara when she was fighting with intent to kill she held her own quite well and her Byakogou seal made her invincible. Onoki honestly has what it takes to solo several Akatsuki with Jinton, if Gaara were in the desert he'd easily murder most Akatsuki, then you have the Raikage who's the fastest ninja alive.
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