That could be the story. I have an idea that Martians unite under one army and rebel against the colonists on Mars (they were being treated a second-class citizens). Many people are killed. They then head for Fanon, Pluto, Earth, Europa and Titan (and all places with people living on them). The nations unite to stop them.
Europa and Titan already have aliens living on them, the Europans and they help the humans. Here is the timeline.
1940 -- Mars Rebellion.
1940 -- Fanon Invasion.
1941 -- Invasion of Earth, Europa and Titan.
1941 -- The Europeans and the Europans sign treaty stating that they will help each other defeat the Martians. This is called the Titan Alliance. On Fanon the United Countries signs a deal with the other nations and forms the Union of Defence.
1942 -- The Titan Alliance and the Union of Defence agree to aid each other.
What happens next? You decide.
In the mean time do say role play and zap some aliens.