Planet Defense (SEV)
As your empire grows, you will generally encounter at least some military conflict. Whether you instigate it or are the target of another empire, a fight will probably occur and at some point, these fights will move into your own Systems and Colonies. The most obvious way to counter these attacks is by having enough ships to fight off the intruders. The game also has a lot of other options for surviving combat that has reached home, sometimes stopping it entirely.