When you attack Invaders and Uber Invaders, you spend Hero's Energy. The stronger the opponent, the more energy that is required in order to attack. 

You can restore your Hero's energy by:

• Waiting for a certain amount of time, until it is restored to its base level automatically
• Using the Hero's Energy item
• Leveling up your Hero, which restores energy to the base level automatically

Hero's Energy items can be obtained by:

• Buying them in the Item Store for Gold
• Purchasing them as part of Bank offers
• Receiving them for completing Quests
• Receiving them as a reward for waiting for maintenance to be completed
• Obtaining them, along with resources, in resource locations
• Finding them in the Chest of Loki