My question stems from this post by p.marino, as it raised my curiosity. I do not wish to spend the time and money to acquire and read Silhouette Core. Furthermore, I lack the knowledge of piles of game systems, so I would not be able to extract the essential characteristics. But some of the more hardy folks here might be!
What are the essential characteristics of a game system that can represent from one-on-one duels (tracking every wound), scaling through skirmish-level engagements (say, scores of combatants in a multi-hour firefight), to large-scale battles with multiple thousands of combatants?
What are some systems that were designed with this in mind, where it wasn't added on as an afterthought (like D&D)? (Preferably, I'm looking for systems available free in PDF.)
I realize at least one of the answers is going to be "use GURPS." Even then, the information I seek is, "What makes it so versatile?"
Related:
Combat rules for large battles
How do I quicken / sum up larger (but not army-scale) d20/DnD3.x combat?
(Any and all help is appreciated in improving this question to better match its single tag.)