Divide and Conquer
As Earth slowly began to expand to the rest of the solar system, it began to tighten its rule on the people of earth. Between dwindling resources and a dying planet, The “Earth Empire (Delta Team)” began to make unreasonable demands on the colonies, demanding more and more exports to meet their needs. Eventually, the outlying colonies (Echo Protocol) began to revolt, with even the soldiers joining in. The EE began sending fleets of units to the mediary planets, tightening their grip on them. In addition to that, the EP began invading those neutral colonies. Not wanting to be caught in the middle of a war, the middle planets formed a coalition to fight the other two factions (Alpha Core). Thus, there is a 3 way war for independence and control. Each faction has sympathetic generals with different preferences, thus the difference in units.
The PCs are part of a unit that is on the forefront of the assault. They will take part in a war that will determine which faction ultimately will reign supreme.