In the beginning we're just going to get our feet, but things will pick up soon and we'll have to do a bit of diplomating with alien factions.

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I can't remember if this is canon or not, but the Andalites aren't sold on the whole 'war' thing, and if they are, they're not too sure about saving Humanity. Officially anyway, but expect persons like Elfangor to show up to help.
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The Chee will make an appearance, as will rebels and other aliens that can be freed from the Yeerks like the Hork-Bajir. Maybe even aliens from outside Earth that haven't been targeted by, or escaped attacks by, the Yeerks.