Humans won the war against the Blues, thanks in large part to the neural link they stole from the aliens. Few people can use it properly, though, and anyone with the right kind of brain gets conscripted immediately -- even ordinary high school kids.
All Sophie wanted to do was be a dancer. She definitely hadn't planned on piloting a sixty-foot ANGEL mech with only a cranky rabbit mechanic to talk to, or fighting the genetically engineered foxes...