As per usual I'll suggest HERO System.
· Active defense as the default. You cannot defend yourself passively, and normal defense is not hand waved as a passive mechanic
- Ability to parry and even counter-parry, and riposte
HERO does this. Basically, the player can place the character’s combat modifiers in any of: defence, attack, damage. These combat modifiers can be re-assigned every time the character gets an action. They can be split between attack, defence, and damage as the player desires. (There’s more to it than that, but that’s the basic idea.) BTW, the skill modifiers are called “combat skill levels” in game jargon.
Parry and counter parry are handled by the manoeuvre called “Block.” It’d take up a bit of space here to go into detail. But it does parry/riposte/parry quite nicely.
· Different styles matter.
- You don’t use a later French sword for a Spanish Destreza style, you use a longer blade
- Different styles, stances, signature moves, make a difference, and there are trade offs for mastery vs versatility as one style may be better in rough terrain or against a larger opponent
· Different attacks are different. You don’t defend the same against a strong high attack vs a low quick attack, and you use the same in different situations.
Out of the box, HERO does NOT do either of these. It’s more abstract and is covered by where you place your combat skill levels. There's nothing to model slight differences in blade.
You CAN build a system to model this sort of thing, but it'd take work. About 10 years back I modelled all the main fencing styles from 7th Sea. Still didn't take into account the differences in blade.
· Momentum matters. When you are on the back foot you need to do something to change that and then try to reverse it.
- An action to regain the initiative/position/whatever should be distinct, and also not a death spiral enabler
HERO does this. By utilising the Speed mechanic and a move known as “abort to a defence” a character can go from being on the back foot to attack. A character’s Speed dictates the number of actions a character can take and the order in which they go, BUT players can hold actions to take them later. Abort to a defence is to take a defensive action out of turn. The Block manoeuvre I mentioned above can change initiative. There’s also the chance of a combatant being stunned and losing actions recovering from that. It's very easy to use this all in play to change the momentum of a combat.
· It’s still a game. The players shouldn’t need to know who Agrippa is to become a master diestro and utilize his ideas. The mechanics shouldn’t rely on fighting knowledge, but should teach what the players need to know, and use game mechanics to model physical actions, panache, trickery, intimidation, etc.
Well, as I mentioned above, HERO keeps this sort of thing abstract. No external knowledge required. So, yes, very much a game.