At The Warrior's Mercy by Denise Lynn

Married–by order of the king! 

Deceived and alone, Beatrice of Warehaven is forced to flee–straight into the powerful arms of feared warrior Gregor of Roul. He escorts her home, though not before a kiss ignites true passion between them. 

If Gregor is to gain his freedom, he must obey one last royal order–overthrow Warehaven and marry Beatrice. His betrayal will earn Beatrice's hatred, but Gregor is prepared to go into battle with this stubborn beauty–and finish what he started with his innocent bride!