Two large and powerful space empires are currently engaged in a very tense cold war type stand-off with each other. The Colonists are a republic that is concerned with the safety of their home system. The Outsiders are an empire loosely based on the Roman military and government, who want to expand out of their current system and establish a full colony in the Colonist system. For the most part, neither side wants to break the very uneasy peace, but the magnitude of weaponry at each side's disposal means that the first shot of a war could also be the last.
Making matters worse, there is a group of raiders, pirates, outlaws, and slavers that have been united under a single ruler that was once a commander in one of the Outsider empire's fleets. This group, collectively known as the Butchers, is trying its best to cause an all out war by any means necessary. Though the Butchers don't have the technology, resources, or numbers to take on either side directly, they are ruthless and use subterfuge, sabotage, and hit and run tactics.
Arms have been built up on both sides. Ships, guns, ammunition, and manpower have all been acquired in case war does break out. Both sides of the war have similar resources and manpower, with the Colonists having a very slight edge in terms of technology. Both have FTL travel, but special Gateways are the fastest, easiest, safest, and cheapest way to get from system to system. These Gateways cannot be built by either empire, and cannot be removed from their orbits. These two empires have fought before, and small skirmishes happen from time to time, but these are becoming larger and more frequent, and fighting happens in space and on mining platforms on distant asteroids and comets.
So how can I keep this situation from escalating to an all-out war between the Colonists and Outsiders? I've thought of a kind of Dead Man's switch that would cause mutually assured destruction, but considering the technology level of these two empires I don't think that is very practical. So what I'm looking for is, for lack of a better word, some more interesting reasons as to why these two space empires would remain locked in a stalemate.