Crossing the Line by Karen Traviss

Shan Frankland forever abandoned the world she knew to come to the rescue of a lost colony on a distant and dangerous planet -- a hostile world coveted by two alien races and fiercely protected by a third. But in the course of her mission, she overstepped a boundary and stumbled into forbidden lands. And she can never go back -- to being neutral, to being safe. To being human.

War is coming again to Cavanagh's Star -- and this time, the instigators will be the troublesome gethes from the faraway planet Earth. Former Environmental Enforcement Officer Shan Frankland has already crossed a line, and now she is a prize to be captured ... or a threat to be eliminated. But saving a coveted world and its fragile native population may require of her one unthinkable sacrifice: the destruction of her own ruthless, invading species.