A turn-based grand strategy game in space: Defend Earth against an alien invasion from the edge of the Solar System. Negotiate with the various governments of Earth and secure their support - One way or the other. Establish your bases on other planets to take the fight to the enemy. Design and build powerful spaceships, then take them into battle.
Here I'm sending one of my fleets to attack an enemy base. Planets with a lot of moons will prove a challenge to conquer, since your fleet will likely be damaged after taking the first beachhead and the enemy can counterattack quickly.