Once war began, there was no turning back. Kumogakure ignored the other Shinobi Villages and, after probing Konoha and confirming that Hashirama Senju was dead, became the first to launch a war. At the same time, to keep Uchiha Mu from reading too much into it, they spent what little money they had on goods from the Hundred-Clan Shinobi Nation.
Uchiha Mu was very pleased and continued selling to them at the original prices. He would make a profit either way, and it cleared out all the inventory left over from his earlier decision.
With Kumogakure declaring war, Iwagakure could no longer hold back either. Although Uchiha Mu's missions had eased their burden for a while, they still could barely sustain it, so they decisively declared war on Konoha without hesitation.
The Land of Grass-no, Uchiha Mu's nation-was very pragmatic. It announced that as long as they did not invade its inhabited areas, they could pass through anywhere else. Of course, even in the former Land of Grass, passage was all they could allow, unless they exposed Uchiha Mu as the man behind it.
Then Uchiha Mu announced that anyone who damaged the railway connecting the Hundred-Clan Shinobi Nation to the Land of Rain would be considered to have declared war on him. That would also make them exercise some restraint in the Land of Grass.
Iwagakure took this very seriously. Officially, they were merely passing through the Land of Grass and would absolutely not damage its railway. As for why there was a railway there-please, did the strong need a reason to exact tribute from the weak?
After Konoha learned of this, it also instructed the ninja forces heading to the Land of Grass to fight Iwagakure to pay particular attention to the matter. They absolutely could not give