This is "The Crucible of Empire" by Eric Flint.
The book opens in the Rocky Mountains after an invasion (by aliens with tusks).
Conquered by the Jao twenty years ago, the Earth is shackled under
alien tyranny - and threatened by the even more dangerous Ekhat, one
of whose genocidal extermination fleets is coming to the solar system.
The only chance for human survival is in the hands of an unusual pair
of allies: a young Jao prince, newly arrived to Terra to assume his
duties, and a young human woman brought up amongst the Jao occupiers.
But, as their tentative alliance takes shape, they are under pressure
from all sides. A cruel Jao viceroy on one side, determined to drown
all opposition in blood; a reckless human resistance on the other,
which is perfectly prepared to shed it. Added to the mix is the fact
that only by adopting some portions of human technology and using
human sepoy troops can the haughty Jao hope to defeat the oncoming
Ekhat attack - and then only by fighting the battle within the sun
As you've mentioned in your comment below, the book was preceded by "The Course of Empire",