The root cause of the war was resources but that was obscured by a broken treaty and the death of a diplomat.
Space is big but man’s ability to travel it is not. Resources need to be found in places which can be reached and from which they can be retrieved at the least possible cost. Habitable planets are the jewel in the crown of any star empire but getting all the resources needed to build and maintain a star fleet out of the accompanying gravity well is a costly business. That makes certain types of metal and mineral rich asteroid belts very valuable. Worth fighting for. A clustering of systems with such belts even more so.
After the initial disputes about jurisdiction, allegations of claim jumping, demands to be protected and the resulting military skirmishes, there was a treaty negotiation. The treaty was fair, equitable and took a great number of intelligent, resourceful and well-meaning people a great deal of time and effort to negotiate and get ratified.
Then Merrick Minerals, a wholly owned subsidiary of Merrick and Sons which was itself a wholly owned subsidiary Lansborough Family Holdings, set up an unlicensed mining operation in a system ceded to the treaty party it didn’t belong to. It complicated this manoeuvre by eliminating the prospecting team sent in by Chung Mining who did have legal rights to the area in question. Merrick Minerals relied on the Lansborough political connections to protect them from the consequences of their multiple illegal acts, that strategy having worked for them before. However, that protection gave the appearance that their actions were government sanctioned.
During the subsequent peace talks in the offended nation’s capital one of the visiting diplomats was attracted to a local woman from a family with some rank and social standing. He made perfectly proper and acceptable approaches to her and her family in the correct local form. However she had other suitors and one of them decided to improve his odds of being the successful suitor by eliminating several of his rivals. Unfortunately for the negotiations, the frank and unabashed assassination of a foreign diplomat in front of his delegation by a member of a local ruling class didn’t strike this ardent gentleman as something which might cause problems.
The peace process broke down at that point and war ensued.
The lady and her family chose a suitor who actually had both political nous and common sense.