Ground combat on Eris against the Separatists:
September 15th, 2054
General Stephanie Forster and her Hovertank Division Alpha, the first brigade of Hovertank Division Beta, a brigade of Garrison troops, and 11 replacement battalions, successfully land on Eris. The big and vulnerable Broadsword class Troop Transports move back 5m km from the planet's surface because of rumors of short-range anti-ship missiles held by the separatist army. Two Abjurer class Area Defense Cruisers remain in low orbit to respond to any threats.
Enemy forces include 8 conventional armor divisions, 16 conventional infantry divisions, 1 mobile infantry battalion, and 2 garrison battalions.
So the Separatists have over 24 divisions and the Commonwealth has about 2 divisions, but separatist forces are almost entirely low-tech conventional units. Militia forces, really, with tanks constructed from heavy tracked mining vehicles and fitted with laser drills. Meanwhile the Hovertank forces represent the pinnacle of Combine technology. They have the best commanders, the best training, and the best equipment. Forster also wants to make an example of the Separatists both to dissuade other colonies from declaring independence and to ensure funding streams for ground forces that are often ignored in favor of Fleet Command.
Eris is an interesting and hazardous battleground. It's just 1/4 of 1% of the Earth's mass, and less dense, honeycombed with artificial and natural underground caverns and facilities. It also contains a population of about 1.3 million humans.
Eris has an escape velocity of just 1.38 km/s, less than the muzzle velocity of a conventional rifle, and far less than the Gauss and railgun weaponry used by the Commonwealth. Traditional "Over the horizon" artillery attacks aren't possible because of the weak gravity, and the horizon is always surprisingly close on all sides. The anti-gravity capability of Hovertanks is carefully calibrated by engineers to keep the armored and shielded units fast and nimble, but well-grounded.
Both sides experiment with making direct attacks through the planet itself, targeting enemy units over the horizon by firing through the rock of the planet. Occasionally, this is effective and occasionally it his and destroys critical underground infrastructure or mining facilities under the surface.
It quickly becomes obvious that the "front" in the engagement can be fluid. Flanking maneuvers cutting a different way around the planet's surface, or even straight through the center are common for the separatists. They're occasionally able to isolate the comparatively vulnerable replacement battalions and inflict casualties before Hovertanks can come to their aid.
As the battle wages on, the fatwave video broadcasts from Eris, long laughed at by people on Earth, become filled with unfiltered images of wildly successful Hovertank Divisions... and grim and up-close images of the civilian and military casualties on the side of the Separatists.
These are not faceless robot battalions from a vault under the red surface of Mars. These are the sons and daughters, friends and colleagues of Commonwealth citizens.
Media on Earth begins to play footage of the alien attack on the corporate transport en route to WX Ursae Majoris that killed 50,000 humans in cryogenic sleep - a terrible atrocity committed by an alien intelligence these separatists sided with, but images of death don't cancel out images of death in the minds of Commonwealth citizens.
The fighting is bloody and many citizens and politicians of Earth call for it to stop, but the Separatists do not surrender.
They truly believe that the Commonwealth is doomed to fall to the Ancients and to again fall under the banner of Earth means doom for the people of Eris.
After 3 months of fighting, all Separatists ground forces are destroyed. The Combine has only lost the equivalent of about 1 full battalion of replacement troops and many of its units and commanders are now grizzled veterans.
When the armistice is finally signed, only 480,000 colonists are still still alive out of the original 1.3 million.
The military has successfully showcased its abilities, but military and civilian leaders are now under immense scrutiny for their inhumane actions, something that will weigh heavily in the next elections.
Resources reabsorbed into the Commonwealth include 7 construction factories, 6 conventional industry, 18 auto mines, 1 GFTF, 2 tracking stations, and a total of 44,000 tons of TN material.
The training facility is immediately put to use building a local garrison out of Commonwealth loyalists and 2 Apollo freighters leave Earth orbit to bring out 2 mass drivers to start bringing these materials back to Earth.
Most people in the Commonwealth seem hasty to put the horrors of Eris behind them. Some redouble the efforts to colonize distant systems and leave memories of the Sol system behind.