This Shattered World by Amie Kauffman & Meagan Spooner
Lee is captain of the forces sent to Avon to crush the terraformed planet's rebellious colonists, but she has her own reasons for hating the insurgents. Rebellion is in Flynn's blood, and he is determined to avenge the children who were let down by the terraforming corporations. When he catches Lee, he returns to base with her as prisoner, but as the rebels plan her execution, Flynn gets second thoughts. He and Lee quickly escape together, caught between two sides of a senseless war.