A Hymn Before Battle is military SF, with nasty aliens out for interstellar domination (the Posleen) versus several varieties of peacenik aliens who in desperation (they’re losing badly, imagine that) bring Earth into the conflict. Humans have been waging war for some millenia, and the armies of the world aren’t about to roll over and let the Posleen harvest humanity without a fight.
The Posleen War is the series. The battle starts here. I enjoyed A Hymn Before Battle tremendously, and here's a wish that you will, too. It’s available for free at Amazon, and from the Baen website. Not sure why, but they’re charging for the book on B&N, Kobo, etc., so be sure to get it from Baen or Amazon, okay?
Thanks for the read, Baen. It’s been thoroughly appreciated! Cheers,