In Season 2 of The Expanse, while Sergeant Draper's ground team is being attacked, a fight between Earth and Mars breaks out in orbit above them. I am confused as to what happened to cause them to attack one another.
A lot of this goes unmentioned (probably due to lack of time) in the TV series but essentially the battle was caused due to the attack by the proto-moloecule warrior created by the Protogen Corporation and a misunderstanding of the situation by Earth/UN and Mars forces.
The situation is explained a little more in the novel Caliban's War where it established that the proto-warrior attacked UN forces on Ganymede and as they tried to escape from it the were engaged by Bobbie Draper's squad who thought they were under attack.
Both sides reported being under attack and both assumed that the other was the aggressor.
It's not stated how this expanded into conflict in the space around Ganymede but one can imagine both sides accusing each other, both denying responsibility and eventually someone pulling a trigger and the conflict escalating from there.
The actual point of how the fighting expanded is not fully covered in the novel but is rather taken as a fait accompli and the primary dramatic issue in the novel is how the two sides settle the matter and move on rather than finding out how.