Not to be critical, but on this forum there appears to be a perception that combat is to be resolved with numbered dice, as evidenced by the recent stream of people asking for help with dice-based combat games. While that's not a bad place to turn for combat systems, often they end up far too convoluted for the average (non-wargamer) player.
Of course, if wargamers are your target audience, then you can ignore this thread.
However, there is a wide variety of possible mechanics that you could use to simulate combat. I want to make this thread a place for people to share mechanics they've come up with, as well as mechanics that can be found in successful games. Here are some examples:
Game of Thrones gives each player 7 or so faction-specific cards that form a hand, each of which has a value that you add to your unit count to give your combat total (if it's a 3v1, the player with three units could still lose if the opponent uses their +4 card while they only used their +1). In that game, all units would route unless the winning player had swords on their combat card, which could be cancelled by towers on the defender's card. Once a card is used, you can't use it again until you play through your hand, so you have to choose carefully, and other players can watch what's been played to get an idea of what's coming.
In Elder Sign, to defeat a monster, you must roll certain symbols on their custom dice. You are allowed to re-roll as many times as you like by subtracting a dice form your dice pool, forming a push-your-luck style combat, since one dice result removes an extra dice from your available pool!
Sid Meier's Civilization (Not the Avalon Hill Civ that everyone raves about) introduces combat cards that are drawn and played in a strange RPS format. The attacker plays their first unit, and the defender in response can play a unit either in front of that unit (challenging it) or they can place it to the side starting a new "front". You each take turns playing units to attack each other until you run out of cards. Cards are sometimes damaged or removed, depending on the Rock-paper-scissors (with air trumping all), so you can reduce your opponent's combat score overall by removing their units, or play more units on other fronts to raise your own score if you can't defeat theirs. Heavily criticized, but if executed a bit differently (and not in a 4-hour game) it might work well.
What other unique combat systems have you seen in games?
