The Arrival (Animorphs #38)

Andalites have finally arrived on Earth. Ax and the other Animorphs now have allies willing to fight against the Yeerk invasion. But there are only four Andalites. Not nearly enough to defeat thousands of Yeerks. Not enough to stop humans from being infested. Everyone agrees that the battle must continue, but the Andalites don't want to fight alongside humans. They feel their skills will be less than adequate. And they demand that Ax choose a side. Will Ax stay with his friends... or stand with his people?