In Her Name Legend of the Sword

In the second book of the In Her Name: The Last War trilogy, six months have passed since the destruction of the human colony on Keran by the alien Kreelan Empire. Earth and other human worlds band together to form the Confederation of Humanity to provide a mutual defense against the alien invaders.
 
Unfortunately, not all human worlds want to join the Confederation. Some, like Saint Petersburg, would rather see it destroyed. With a powerful navy built in secret and armed with nuclear weapons, Saint Petersburg is preparing their own offensive against the Confederation when the Kreelans attack.
 
Led by Tesh-Dar, an enigmatic Kreelan warrior priestess, the Kreelan Empire has begun its quest in earnest to find the One, an alien who might be able to lift the ages-old curse that will leave her species extinct in but a few more generations. But to find the One, they must bleed the humans in war...
 
If you're new to the In Her Name series, here is the author's recommended reading order:
 
1. The Last War Trilogy

• First Contact
• Legend Of The Sword
• Dead Soul

2. The Redemption Trilogy

• Empire
• Confederation
• Final Battle

3. The First Empress Trilogy

• From Chaos Born
• Forged In Flame
• Mistress Of The Ages

Note that The Last War and Redemption have also been published as trilogy collections.