Elf: But this is immediately after a brutal war to kick Benignon *out*. I can certainly buy the darker elements of Daien being willing to fight laguz, sure, but to help their hated rival who defeated & occupied them 2 years ago or so? It feels like, I dunno, 1920 Germany being asked to send troops to help out the French. Even if it's a really hated enemy, we're kinda busy rebuilding over here to help out a slightly-less-hated enemy.
Anyway, I checked the script, and I guess we're both sorta right. Okay, Pelleas does mention the sub-human thing and that it was a request from Benigon. I was more referring to this part, as well as the chapter title of "A Reason to Fight":
--
Micaiah: The Daein Royal Army will answer Begnion's request and attack the
Laguz Alliance at the Ribahn River. Those are our orders from King Pelleas.
As his subjects, we will obey him.
Sothe: Without even asking why? Daein is finally on its own. Why are we even
speaking to Begnion, let alone fighting for them?!
Micaiah: I don't know. I do know that King Pelleas wants what's best for
Daein...just like us. He must know something we don't. I trust him. He's
changed since the coronation, though. He doesn't smile or laugh anymore.
Maybe it's because Izuka and the Black Knight vanished.
Sothe: Who cares about Pelleas smiling or laughing? We have no quarrels with
the Laguz Alliance! Why are we targeting them?
--
From the dialogue, it's pretty clear that Micaiah didn't pry into anything deeper than "Benignon asked," with Micaiah assuming that maybe Pelleas knows something. And Sothe (and SnowFire!) don't think this is sufficient and say WTF. Maybe Pelleas does know something, but... he should tell you it, then. (Yes, yes, I know that the script doesn't want to reveal this particular plot point yet, but the characters don't know that.) In fact he's going to have to tell lots of people if he wants them to risk their lives, some sort of good reason. So yeah, there's a reason, but not enough of one for my tastes. It's like saying that Batman will fight Superman because Lex Luthor told him that Superman killed his parents. Okay, fine, but people say lots of shit, especially villains, we need more if we want to buy this confrontation without reflecting poorly on Batman's gullibility.
To give RD some credit, I'm willing to throw fiction some breathing room to set up OMG dramatic confrontations between two sympathetic forces, because people, including me, love that. And I thought that among RD C1's best plot points was reflecting that Daien's rebellion was an alliance between its noblest and worst parts, what with the BK & Izuka helping you out. RD C3, though... precisely because it was a well-developed world, it annoyed me how it became nonsense because of WHOA you unknowingly signed a blood pact and don't think too hard about it. Only when I actually care somewhat about the setting & characters will I sit up and say "No, Zihark is not going to go along with this without a REALLY GOOD REASON, penalty flag." (And hey, there is a way to send DB characters back to Ike, if you hate yourself! So there's that at least, a nice touch there noticed how out of character this was for a crew that went around saving imprisoned laguz earlier.)