For every battle, you’ll have to meet the specified Victory conditions to clear the mission. At the same time, you’ll fail the mission depending on the specified defeat conditions as well. If ever you mess up, you have the option to use a Divine Pulse charge to turn back time and roll back your actions to a certain turn, in order to fix the mistake. Otherwise, you’ll have to restart the battle from the beginning.
You can view the Victory and Defeat conditions by selecting the Map option in the Battle Preparation screen. From there, press the “-” button twice to zoom out and display the map, conditions, and unit count as shown below. You can do this even during the battle.