Once you've deployed troops, there's no turning back for them. This was put in place to make you think carefully about the tactical aspect of the game: what is the minimum amount of troops that I can use for the best possible outcome? As long you don't deploy your troops, they will return to your camps after the battle.

Try practicing your attack strategies in the single player campaign. This way, you can see how different troop load-outs work for you without losing troops unnecessarily! Experiment with different techniques and troop combinations. Each unit has its own unique strengths and weaknesses.