All troops deployed on the battlefield remain behind to hold the conquered island against enemy counterattacks. Expending troops to win battles is at the heart of strategy in Battle Ages. Think of your troops as resources to be spent wisely - you should plan every battle carefully and try to use the right number of units for the right situation. Throwing all your men into combat without considering the cost can lead to a pyrrhic victory, where you end up spending more than you win!
(See here for Pyrric Victories throughout history: https://en.wikipedia.org/wiki/Pyrrhic_victory)