Let me begin by stating, that it's not important what you fight, but what you fight for. In Mouse Guard you most often fight the natural enemies of the mice, such as snakes or weasels, or you fight more abstract things like corruption or bad weather, but in the end, you should fight in order to protect the Mouse Territories, the order of Mouse Guard and your fellow guard mice. Because when you're all small and weak, the best way to survive is to stay together.
Mouse Guard is an original comic series imagined by author/illustrator David Petersen who has converted the comics to the Burning Wheel role play game system, published by Archaia, together with Luke Crane, the designer behind the BW system. I have yet to acquaint myself with the original comics, though I have them lying around somewhere. Also, I've only played MG two times, once as a player and once as a game master. Even though I've seen the game from both sides of the same coin, I have hardly the experience to give a definitive analysis of it; nevertheless, I will attempt a review of the game. This I do mostly for my own pleasure as well as an incentive for future players of MG.
I myself have played with, oh, dozens of role playing game systems, I suppose. But BW has so far remained a mystery to me, even though I've glanced through some of the books occasionally. MG RPG I received as a Christmas present (or was it birthday? can't remember...) a couple of years ago, but my busy schedule with other games has so far discouraged me from trying it out. I was finally able to play a one-off session of it when I visited the Conklaavi gaming event in Turku, organized by the Turku University roleplayers, Tyrmä ry and this summer, in Oulu University roleplayers' gaming event, Maracon, I was able to host a game of it. I used one of the ready-made adventures from the book and the characters from the original comics - in Turku we played a completely home-made scenario with pre-made characters the GM had designed specifically for our adventure. Both times I enjoyed the game very much and both times I found that the game worked even when both the GM and the players were a little lost at the general rules and concentrated rather on the storytelling and throwing the dice when a conflict arouse.
The system is pretty straightforward - if there's a need for a die roll, you roll. This can happen for instance to check how well something works out, or how much time doing it takes. What happens is always decided after the actual die roll. And even if the die roll is not good, the character can incur penalties to make their action succeed, or to work out. So failing can be pretty rare in this game; although the costs can rack up pretty quickly. Say for instance, that you're looking for something out in the wilderness, a place to rest, another guard mouse, a stream with fresh water. You see your mouse's stats and skills and choose which ones to use and if you have any wises that can help with your throw, or if your party members will cooperate in this action. You then get the amount of d6 dice that your stats and skills show, as well as possible bonus dice from wises and friends. You try to roll fours or better - these are successes - and to get as many successes as possible. If you fail, you can negotiate with the GM. Possibly your mice find the stream, but it takes them such a long time that they are tired, or angry, by the time they get there. The conditions always give you negative effects and steal away the dice you can use on checks and can only be healed during the Players' Turn.
If there is no need to a check, you just tell the story. Here GM and players can cooperate to advance the plot, the players can ask questions or even suggest things to happen - but the main role is on the GM's shoulders at this point. Whenever a more dire situation occurs, the players begin to resolve a conflict, usually against the GM, but they can sometimes occur among the players themselves as well. Before a conflict, the target disposition will be calculated for both parties - this is the number which you will then start to play against. The difference between the enemy and the party successes either increases or decreases their respective dispositions; so for instance if a guard mouse attacks with five successes and a weasel defends with two successes, the weasel loses three disposition. Beside attack and defend there are also feint and maneuver actions, which either give you an edge in your next roll or force the enemy to forfeit their defence completely - these are all better understandable when playing the actual game. In a conflict you need to decide three actions in advance and in secret - these are then revealed one by one and resolved against your opponent's actions. Here one is required to consult a table to see what effects each action has when combined with another one; it is difficult at first, but once you get the hang of it, playing the conflicts will get a lot smoother. The conflict ends when either of the target dispositions reach zero - the one to get there first is seen to have lost the conflict.
Once the main conflict of a given session is completed, the players get some time-off. During this time-off (called the Players' Turn in the actual rules) the characters can heal up, keep up their contacts, get new equipment, level up and so on and so forth. Use their free time whatever way they please.
So that's pretty much the actual game play, with a few simplifications here and there. I myself like it for its simplicity and how its not overdependent on die rolls. But then again, I like it when there's room to maneuver between what the character can do and they may do. The skills and wises can be expanded a lot to cover this and that aspect of the game, and if you don't have a skill that would fit, you can always roll your Nature which functions as a basic die pool, as well as gives a notion of how mouse-like your character is. Your Nature may, as per other abilities and skills, go up as you play, but it can also go down. If it goes to zero, your mouse leaves play, as it becomes no more than an ordinary field mouse, having lost the human-like aspect of itself that made it able to be a guard mouse.
Apart from abilities, traits, skills and wises, which all affect in your game play mechanically, each character also has three other important plot devices: their individual Goal, Instinct and Belief. Goal is written up before each adventure and must somehow apply to the current mission the guard mice are on - it need not be exactly the same as the mission outline, at least not with all of the mice. Accomplishing a goal generates a Persona Point, which you can use to add one die to your pool or to tap Nature to any one roll. Doing this Nature tap, however, lowers your Nature score if the roll is outside your Nature (meaning, does not directly relate to it somehow) or if the roll fails. Instinct is exactly what it sounds like - what are you likely do to in a given circumstance. For instance, Saxon, a mouse from the original comics, has the Instinct: "I always draw my sword at the first sign of trouble", while Lieam's is "Always offer help when needed". Playing an Instinct earns a Fate Point, which allows you to reroll any sixes as new dice, meaning more successes. Instinct is a more permanent trait with each mouse, although it can of course be changed over time. Belief is a little more complicated. It is like a world view, or agenda, or general attitude to life, that your mouse follows in most, if not all aspects of its life. Changing a Belief requires serious character development to happen (if you follow the rules rigidly) and playing it earns you Fate points, as was with Instinct. All of these points are earned at the end of the session, to be used in game next time. Why I like these three guidelines, is that they give you a sort of a 101 course on your mouse first, and a mechanical advantage second. Not to mention, they can also have disadvantageous effects, in certain circumstances.
When you begin to play a new scenario, one of the players should recap what happened last time. This allows for everyone to catch up, but it also shifts from of the responsibility off the GM - and also, the storyteller may remove one of his effecting conditions or recover a point of Nature he has taxed in previous session.
So that's it for the mechanics. I like the mechanics, even though the dice pool system is sometimes taxing, in that there are times when you get no successes and times when you roll all sixes - and sometimes you roll right according to the probability curves and all mathematicians squeal with joy. The main point is that the mechanics are simple, easy to learn and fun to use. Even the conflicts are fun, when you get used to them. Also, I like the character sheets provided by the game - they're big enough for your handwriting and also contain the most important tables and die information, so you don't need to flip through the book all the time during play.
Most of all, though, I like the world. I won't go into it too much, as I don't know so much about it myself. It is one of the rare games that I've played as other than human - not that it's so rare, just that I haven't done it so much. It's interesting to try and relate to problems a couple of mice might have, such as high grass, badger's nest or lack of berries due to dry season - all the while keeping in mind that what you're doing is for the protection of everyone else. So I guess, as we come around the circle, what you fight for depends a whole lot on what you are. Maybe some day I'll create characters for the game from scratch - because that's always my favourite part - and I'll get to really enjoy and appreciate all the possibilities the world has to offer.