Conquerors' Pride

A very good book series about a interstellar war between humanity and a alien race called the Zhirrzh.  A team of soldiers sneak on to the aliens homeworld to rescue a human that is key to winning the war.

There are three books in this series,  the second book called Conquerors Heritage is told completely from the aliens point of view.

And the third is called Conquerors Legacy.  In this book the humans are still barely holding on when a discovery leads them find out the whole wars was based of a misunderstanding.  And that both of the races need each other.