The Laws and Customs of War: There are several sets of these, locally; the Empire has its Imperial Rules of War, and so forth, but the ones people mean in the general case are those contained in the Ley Accords, which might be considered the Associated Worlds’ closest approach to something like the Geneva Conventions.
They come in effectively two parts: Chapter I, and Chapters II-XVI. The former covers “Instruments of Regrettable Necessity”, and nobody screws around with Chapter I, because what it means are the big ones – star-killers, planet-killers, ecocidal weapons, uncontrolled self-replicators, persistent information or memetic weapons, berserker probes, that sort of thing. Weapons prone to affect people well beyond the battlefield, replicate and spread out of control, stay virulent through deep time, or destroy garden worlds and their information-rich ecosystems. Start deploying these, and the penalty clause attached is “every other signatory will drop everything to wipe out your polity – albeit not necessarily its population – right now”.
Of course, what we consider weapons of mass destruction are fair game, and you can throw around all the tactical nukes, non-persistent chemical and bioweapons, incendiaries, non-persistent info- and memetic weapons and nanoweapons you can carry with merry abandon.
Chapters II-XVI are the Conventions of Civilized Warfare, which cover the usual things – no indiscriminate planet bombardment without inviting a surrender first, no orbital bombardment or demonstration strikes on civilian areas without a military purpose, no using those permitted weapons of mass destruction near civilian areas without a military purpose, no using infoweapons that would corrupt noetic backups (causing Final Death), no mistreating of prisoners (including various kinds of mind-probe as well as torture and the usual indignities), no terrorism, hostage-taking or asymmetric attacks on civilians, using civilians as shields is forbidden, uniforms to be worn, honorable surrenders are to be accepted, quarter to be given, private property to be respected (no foraging or pillaging of civilian volumes), and a baseline standard for treatment of POWs (including communication of capture and subsequent communication with home via the Accord) and civilians in occupied areas, and so forth.
Notable differences from our laws of war include:
- There is no requirement for a declaration of war; attacking is sufficient. Nor do they forbid an aggressive war, as long as the Conventions of Civilized Warfare are followed.
- Parole is permitted. (In some cases, this has led to odd wars in which taking the enemy prisoner was prioritized over killing the enemy, because a parolee could not return to the war, whereas a reinstantiation of a dead soldier could be back at the front very quickly.)
- Spies, assassins, and saboteurs attacking governmental, military, or militarily useful targets are considered legitimate tools of warfare and are protected by the Accords. The fuzziness between some sabotage and terrorism has, of course, given space-lawyers a peck of trouble in the past.
- Mercenaries and privateers are also considered lawful combatants; mercenaries in particular are permitted to exercise their parole option almost immediately as long as they do not return to either side of the war in question.
- Medical personnel are protected by the Accords and may not be attacked (but see Combat Medic); but chaplains are not, which may have something to do with the Empire’s two war gods and their enthusiastic templars. Or the kaeth. Or… well, several others.
- No types of weapons are specifically forbidden other than those covered in Chapter I and the situational limits on WMD use in the Conventions, i.e, fire, poison, blinding lasers, explosive shells, bioweapons, devouring nanoswarms… all allowed. As they say, war’s not supposed to be humane, dead is dead, and no-one’s doing anyone any favors by prolonging it by fighting with one hand tied behind their back.
- Prisoners may not be compelled to labor, even in non-military roles (the Empire hates slavery, and insisted on this clause), although they may be asked to volunteer to do so.
Also, while the Ley Accords are, like the Geneva Conventions, a reciprocal treaty – well, technically, Chapters II-XVI are a reciprocal treaty; Chapter I is applied ecumenically – this is done much more in practice with the Ley Accords. Signatories are under no obligation to respect the Accords vis-à-vis non-signatories who don’t respect them (many do, though, at least until the other party openly defects); and if it can be demonstrated that a signatory is not respecting them (by a tribunal appointed by three nations including the offended-against party), penalties can vary from reparations and individual-level trials for war crimes to the execution of the complete political authority and military forces, in kind as well as in personnel, responsible for the breach of the Conventions. All signatories are required to provide such military forces as are necessary to enforce such penalties.