The Boorman Treaty is an agreement signed by all four Major Houses that forbids direct conflict between the Houses. It is likely that this treaty was made in the aftermath of the 80 Years War in 700 AS. Kusari (rightly) accused Rheinland of violating the Boorman Treaty by building up its military along the Kusari/Rheinland border during the Single Player Plot. Later, in Mission 5, Rheinland blockades the Omega 7 system, which Juni says is a direct violation of the terms of the treaty.
It is unclear how effective the Boorman Treaty has been at preventing armed conflict throughout the Colonies. Indeed, the Houses may continue to fight each other indirectly, through funding and supporting proxies, such as the Gas Miners Guild or certain pirate groups. In light of how major governments have operated throughout human history, this seems exceedingly likely.