Haegemonia takes place in the distant future where humanity has colonized the solar system and tensions are high between the World Government of Earth and colonial Mars. A summit on Earth's moon is planned to ease tensions, but the Martian representative's shuttle is destroyed en-route by a third, currently unknown, party. The two sides blame each other and this one event sparks off the civil war between the colonists and Earth.
The player can side with Captain Jack Garner of Earth to quell the colonial uprising, or join Captain Nilea Cortilliari and the Martian colonists on their campaign for independence.
Once the outcome of the war is decided, be it Earth or Mars, humanity unites under the new Unified Government and forms the Legions of Iron, its vanguard of expansion into the unknown.
As the human race expands outward from Sol, mysteries are solved and new ones are created as they encounter new worlds and alien civilizations.
Source: Wikipedia, "Haegemonia: Legions of Iron," available under the CC-BY-SA License.