Ahh, I just found the rules I had written down that I RP my nations all signed on to.
No genocidal war. Conflicts shall be resolved with smallest scale military action possible, including limited destruction of infrastructure or military assets. Any attempt at the total annihilation of one nation would be met with the declaration of war on the offending nation by all nations.
All efforts must be made to spread the species into space. No nation shall prevent any other nation from following this Law. Since anything could happen to the homeworld or any colony, as many colonies must be created and spread among the stars.
There are Laws governing the claiming, acquisition, and contesting claims of bodies in space. A nation cannot officially claim a body without a good reason. Claims on bodies are split into three tiers.
(
1: Weak claim. No colonists. Some installations, no significant strategic value, or economic gain currently.
2: Decent claim. Small number of colonists or no colonists. Might have some military presence in space or on the ground. Some strategic value or economic gain.
3: Strong claim. Large number of colonists, maybe millions. Permanent military presence in space and on the ground. Large strategic and/or economic value.
)
(These 2 laws do their best to ensure colonisation will be organised but steady. No cowboys running round claiming everything and using that claim to justify military action.)
A maximum upper limit to the total military tonnage that nations could control at any one time. This limit stands at 100,000 tons. Survey ships or auxiliary ships not included.
(I was thinking of raising that one quite a bit, 100K seems a touch limiting. 150 to 200K maybe. I don't know about some kind of Washington style naval treaty to define ship classes to limit or encourage certain things, I'm thinking it won't be a fun idea, so just a tonnage limit early on. At least until no nation needs to care about any other opinion.)
Since the species is supposed to be very much antagonistic towards itself, there would always be at least some tension and potential for fighting.
EDIT: One of the reasons for me having a treaty like that, even though I intended for the nations to eventually abandon it, was to force everybody to use smaller ships. Every nation was using small numbers of sub 10K ton ships, mostly 5 to 7K. I think it is a realistic choice in universe and also lessens the cost of the military and forces economic development earlier on. It also shrinks the scale of the conflict.
Also I remember adding the law about claims after there was a conflict between two nations over colonising a mars like planet in their home system at the same time, so those two nations will always have a place to fight over. I want the various nations to always have a reason to exercise their innate desire to fight.