There was no doubt that Obito Uchiha agreed to Muzan's request and was willing to help him stand against the three great nations, but their task was only to gather the Tailed Beasts.
Judging by the danger involved and the benefits to be gained, Muzan was the one getting the worse deal. He had to shoulder most of the risk and would not gain even the slightest benefit in the end.
Only Muzan himself understood that his purpose was not the Tailed Beasts, but to strike at the three great nations and find a way out of his predicament.
Clearly, Muzan had found one. The solution lay with the Akatsuki, even if an unpredictable Madara Uchiha might appear later.
He was not invincible. As long as no Ōtsutsuki were released, there would be no problem.
Step by step, like a spider, Muzan had unknowingly woven a perfect web. No creature that entered it could escape.
The three great nations did not launch a full-scale invasion either. Instead, they probed Muzan's movements through the smaller nations, watching to see whether he would send reinforcements. Clearly, Muzan cared greatly about those useless allies, and they could slowly wear down the Land of Waves' active forces. In terms of ninja numbers alone, the three great nations held the advantage.
It was only because of the Land of Waves' unique circumstances that they had not resolved to fight to the bitter end.
All that remained was to devise a series of plans. Aside from war, Muzan would do anything that could indirectly weaken his opponents.
The daimyos of the three great nations were assassinated at almost the same time, and the attackers were all men prepared to die.
It gave people the illusion that they had not come to attack, but to throw away