would that also include people who never actually take part in
hostilities, like command personnel stationed at home?
It depends on what you mean by "never take part in hostilities". Article 43 defines what constitutes "armed forces" and the definition is fairly broad. All members of the military's command structure and everyone that reports to them - regardless of rank - are members of the "armed forces" and are considered combatants. This even covers paramilitary groups that are working with the military but not officially part of it. Certain support roles such as chaplains or medics do not fight the enemy, and in some cases don't even carry weapons. They are still classified as "combatants", though, to ensure that they receive the same protections as armed personnel (those specific examples actually have an entire article that gives them additional rights).
If instead you mean people who aren't actively taking part in combat at that moment, there are some special conditions that can apply. When Article 3 is in effect (conflict where at least one party is not a state), it says:
Persons taking no active part in the hostilities, including members
of armed forces who have laid down their arms and those placed hors
de combat by sickness, wounds, detention, or any other cause, shall
in all circumstances be treated humanely ... To this end, the
following acts are and shall remain prohibited at any time and in
any place whatsoever with respect to the above-mentioned persons:
- violence to life and person
- taking of hostages
This covers all sorts of non-combat cases, like troops surrendering on the battlefield, a pilot that ejects from a damaged aircraft, or an enemy you sneak up on while they're asleep. The convention's official definition of hors de combat is:
A person is ' hors de combat ' if:
- (a) he is in the power of an adverse Party;
- (b) he clearly expresses an intention to surrender; or
- (c) he has been rendered unconscious or is otherwise incapacitated
by wounds or sickness, and therefore is incapable of defending himself;
provided that in any of these cases he abstains from any hostile
act and does not attempt to escape.
The general concept is that you can't attack someone who isn't hostile and that cannot defend themselves. A soldier home on leave would likely fit this description, since they left the battlefield, left their weapons behind, and aren't doing anything related to the conflict at all. They're essentially a civilian until they return to active duty. The situation would be different for a commander who was working from home, away from the front lines. They would not be considered hors de combat because performing their command duties would violate the "abstain from any hostile act" requirement, plus would not meet the "[take] no active part in the hostilities" provision of Article 3.
Geography doesn't really matter. The command crew that initiates a ballistic missile launch from a thousand miles away is just as much a combatant as a front-line soldier. There isn't a limit on the org-chart either. Article 43's definition of "armed forces" talks about "units which are under a command responsible to that Party for the conduct of its subordinates" (where "Party" refers to a nation). The Convention requires each signatory nation to have an "internal disciplinary system" that "enforce(s) compliance with the rules of international law applicable in armed conflict." The entire military apparatus is an organization that is subordinate to the national government, thus it is each nation's responsibility to ensure that all levels of the military follow the Convention.
Would he have to don a uniform to make him an eligible combattant,
or come into some specific distance of the front?
Distance from the front doesn't matter, but which side of it you're on does. If he was caught in enemy territory and was not wearing a uniform, the Hague Convention could let him be treated as a spy, subject to arrest and prosecuted under that nation's anti-espionage laws. You would have less protection in that case since it would become a matter of local law and not international law.
