The development of the Hyperspace-Drive triggers a gold rush-like colonization attempt by Earth governments, only to find that habitable (the game takes these as Earth-like) planets are either homeworlds for other sapient species, or colonies of early-evolved species.
Years of build-up and exploration bears fruit: A habitable, rich and lush world with no sentient race is found. However, almost every race intends to colonize it, and soon a full-scale Galactic war will erupt. To solve the bloodshed, the sapient races agree to a competition:
Every race would dispatch a Rocket Ship with a pre-determined size, mass and equipment, stocked with materials, personnel and equipment to create a city analogous of modern Earth technology and infrastructure. Then without interruption from outer space, the base-cities would engage in conflict, with the victor being granted the ownership of the planet. Of course, it is your duty to ensure a new world for your species to claim.
Source: Wikipedia, "Enemy Nations", available under the CC-BY-SA License.