In my earlier post above I think I made clear along which criteria I would define a wargame, but I will spell them out explicitly:
A game is to be considered a 'wargame' if it fulfils all of the following three requirements: (1) The player is able to command and control military units or conduct other military decisions from the position of leadership. (2) The main focus lies on the simulation of some military aspects in or around an armed conflict. (3) The victory conditions reward at least in part the reaching of combat objectives.
A 'wargame' is thus distinct from a 'game about war' or a 'war-themed abstract game' which might fulfil some, but not all, of the above.
Of course, the boundary problem is not entirely solved through such a definition. Having said that, couple of examples: Diplomacy would be in and Twilight Struggle probably out (mostly because of the second requirement, partly because of the third). Roleplaying games are generally out since they do not possess victory conditions, although early D&D (especially with Chainmail, the Sword & Spells supplement, the more recent Delta Book of War) - when the focus of the game is a military campaign - would be in. The COIN series would be in (as well as the fantasy-themed Root), since despite euro mechanics they also quite well succeed at simulation. Combat Commander, though usually considered a wargame, is a more tricky one since it relies on hand management, a metamechanic which arguably does not simulate anything. Compare with Fighting Formations which uses metamechanics, but these simulate the conflicting priorities, insufficient capacities and friction of command and control. Warhammer 40K would be in. Those I would see outside the category would still qualify as 'games about war', Combat Commander probably as a 'war-themed abstract game' (or a 'wargame with added metamechanics'). Perhaps interestingly, I do not think the notion of an in-game opponent is absolutely necessary for a wargame, irrespective of whether it is competitive, cooperative or solo. As an example, Advanced Recon simulated patrols in Viet Nam (see also the Hunters series) and I can also imagine a game based around logistics and campaign planning (which are still military decisions around an armed conflict, even if no actual conflict takes place in the game). This is reflected in the formulation of the requirements above.
