The makers of the movie didn't do a good job of showing that. :redface2: Everyone was pretty miserable and we did not see much of the people outside of the armies. Especially considering all the bujin riders saying they want peace. Plus, Wizard and Fourze's faction teamed up..... So..... WTF?
We did get Ranmaru's speech about how the people were suffering with the dry land, and also Ieyasu's speech about his reasons for fighting and conquest - initially to expand his borders and be able to produce more food for the starving people, and afterwards unify the entire land under him to bring peace.
Alliances are possible if the warlords share the same vision or one of them agrees to become subservient to the other. The movie didn't show how the Wizard/Fourze alliance was made. They didn't even give a name to Fourze's lord. But I don't see why it's so perplexing that some warlords could become allied while others fight against each other. Even if they all wish to unite the land and bring peace, what they'll do afterwards isn't necessarily the same, so they fight. Although, like mentioned above, I guess part of the problem is that the movie seems to assume the viewer has at least some familiarity to the period.
It also doesn't help that Bujin Gaim basically has nothing to do with all that and is just a generic villain, so the movie didn't really give much focus to warlord vs warlord battles, only showing that they existed before Bujin Gaim appeared and started defeating everyone.