Avec Infini Regret II is, like its name indicates, the second volume of a tactical system devoted to the French wars of religion. The two volumes are fully compatible, but this one has the latest version of the rules, so benefits from the feedback of the players of the previous version.
The battles depicted in the system are fairly small, with at most 15,000 men on each side for the largest (Dreux). The counter density is thus low, and the games play relatively fast. The maps are mostly empty, so terrain is of secondary importance, the main focus is on the interaction of the various arms.
The system is a simplified variant of Musket & Pike. The main difference (I think, it's been a while since I've played M&P) is that fire here does not cause hard losses, but a morale check, with a difficulty depending on the extent of the shooter's margin of success. It is a simplification, but one that works very well.
For those who are not familiar with M&P, the core of the system (as I see it) is the orders mechanism. There are three possible orders, charge, maneuver and rally. Each order allows basically what its name says, but the trick is to sequence them. Is is impossible to go from rally to charge, for instance, and easier to rally after a charge than to maneuver. Changing orders is relatively tricky mid-battle (though easier with good leaders), so having a plan from the start (initial orders are free) is important. The activation system is a bit convoluted, but basically formations move by order of decreasing agression, with possible interruptions, which is important when trying to set up charges and counter-attacks.
The game is unabashedly tactical, so facing is important as are interactions between unit types. Each infantry is a mix of shot and pike, with the more pike heavy dominating melee, but vulnerable to being shot to pieces by opportunity fire. Morale is crucial to endure being fired at, to win melees and to rally. The most likely result of melee being an additional level of disorder for both sides, it can pay to send weak , but fresh, units into the grinder if they can push stronger, but disorganized, enemy units past their breaking points.
The chrome is given by the varied combination of unit types and morale.
The Swiss mercenaries, for instance, are formidable opponents, able to withstand heavy fire and having the advantage in melee over most unit types. They can be countered by reiters (pistol wielding cavalry) who, being mounted can not being engaged by infantry, and who thus can shoot the Swiss squares without fearing retribution (as indeed they did historically). The reiters in turn are vulnerable to heavy cavalry, and so on, so there is a distinct role for each unit on the battlefield.
This volume proposes three different battles, two half-maps and a full one. The system is also used for the three battles of the previous module, and two others (Nieuport and Kircholm) in issues of Vae Victis magazine that are, I think, still in print. The small battles are good for learning the system (which is simple, but has many small modifiers and exceptions to memorize), but are a bit too short and can be decided by a few lucky rolls. The larger battles are where the game really shines, with enough units and time to really feel the ebb and flow of battle.
In short, don't let the relative obscurity of the conflict covered put you off - the game provides a very good exploration of the late renaissance battlefield, with a fraction of the complexity of its parent system but retaining its defining features.