Rule Number Four, no biological weapons. This rule wasn't so much to protect the Maxohlx and the Rindhalu, because species were different enough that a pathogen deadly to one species mostly had no effect on others. This rule was more like a Geneva Convention sort of thing; where the combatants agreed not to do something to the enemy, so the enemy wouldn't do it to them. Otherwise, each side could simply drop stealth missiles with canisters of biological weapons into the atmosphere of enemy planets, and soon no one would be left alive on either side.

Rule Number Four was, we heard, a little sketchy in actual use, as viruses, bacteria and other biological hazards mutated over time, and if a known, natural virus became more contagious or deadly, who could say that didn't happen naturally? Rule Number Four was in place to prevent catastrophic biological attacks, but apparently a certain level of cheating was allowed, as long as it didn't get out of control, and no one got caught. The Kristang told us the Ruhar were notorious for pushing the limits on Rule Number Four. I'm sure the Ruhar would say the same thing about the Kristang. The bio weapons thing scared me, despite reassurance from the Kristang that the Ruhar didn't know enough about human biology to make a weapon effective against us. What scared me is the Kristang had medical technology far advanced beyond what humanity had brought to the stars, and they weren't sharing any of it with us. They said that without years of studying human biology, they couldn't safely apply their advanced technology to us. Which meant even a common flu virus, that some human brought with them from Earth, could be a serious problem for the Expeditionary Force. And that wounded troops had to be treated with whatever medical care was available on that planet. There would be no medivac flight back to the States on this campaign.

Rule Number Five was, simply put, no dropping rocks. No steering asteroids or comets so they impacted an inhabitable planet. This also meant no taking even a small rock, accelerating it to relativistic speed, and slamming it into a planet. Any planet with a decent population had systems that could detect and deflect rocks that were naturally on an impact course anyway, the rule was to prevent a combatant from saturating those defenses, or encasing a high-speed rock in a stealth field. What, you make ask, about railguns? Spaceships routinely used railguns to accelerate kinetic rounds up to relativistic speeds, and railguns were used for both space combat and planetary strikes. The key to using railguns for planetary bombardment was the effect on the planet; you had to limit the energy yield of a single kinetic impactor to less than a megatons, and you couldn't use so many railgun strikes that you affected the planet's climate. Pounding a planet with a lot of railguns could throw a thick dust cloud into the air and lead to rapid cooling, that dust could take a long time to settle out of the atmosphere, even leading to a mini ice age.