Let's suppose that Kumogakure attacks Iwa in a similar fashion to how Suna and Oto attacked Konoha.
Period? Start of Shippuden. So there's still tension between all the nations, but Akatsuki still hasn't begun to gather the Jinchuriki.
Similar scenario. Iwa hosted a friendly tournament, and so Kumo shinobi are all here. At some point the 4th Raikage gives the signal, and BOOM, war.
At some point A and Onoki go at it one on one, uninterrupted, until either one of them dies, or the village pushes back against the invaders or is invaded, outnumbering one of the two Kage.
Iwa has access to Yonbi, Kumo has access to Nibi (leaving Hachibi back at the village).
Would Iwagakure be able to push back against their invaders? Would Onoki survive?