Battles and wars are often lopsided. That doesn't mean the result is inevitable, or that just because it did happen a certain way means that that was the most likely result - indeed, it could have been a statistically aberrant one. (For more on this, I recommend reading Brien J. Miller's "The Application of Statistical and Forensics Validation to Simulation Modeling in Wargames" in the book Zones of Control.)
But, for the sake of argument, let's say that we have a conflict we want to model that was definitely lopsided, in which absent major out-of-left-field counterfactuals, one side is very definitely going to win and the other side is very definitely going to lose. The first question you might ask is, why bother making a game on that subject at all? After all, aren't there any number of subjects where the result was in doubt, the sides evenly matched - wouldn't that make for a better game?
Personally, the thing I look at when considering a topic isn't if the conflict was "balanced" or lopsided, but if it is interesting, and further, if the things I find interesting about it can be modeled in interesting ways. Usually these interesting things tend to be the reasons why one side won and the other lost, and because in a lopsided conflict these are often more pronounced, I tend to find the lopsided battles and wars more interesting.
So, here's the next question: how do you make a reasonably competitive game out of it? There are a number of different ways to approach it. Here are three tools that I like to keep close at hand. At first blush they might seem very similar, but each has a different application - in much the same way that a claw hammer, a sledge, and a mallet all have the same basic idea, but each is most appropriate for specific circumstances.
1. Lose the War, Win the Game
I did a game once on the Franco-Prussian War that was eventually published by White Dog. The game was designed to cover the decisive 1870 campaign that ended with the capitulation of Napoleon III, and the game itself was always built to end with that capitulation. That is, the French always lose the war, every single time.
But they might still win the game by suffering a less humiliating and total defeat than was the case historically. In that game, the French earned VP at the end of the game for holding certain cities and for eliminating enemy units, while the Germans earned VP for surrounding French units, cutting them off from supplies, and thus forcing them to surrender. The high score wins the game, and with a good French player, the scores can be pretty close and either side can win.
Both the Germans and the French are awarded VP for reasons that align with historical goals and doctrine. The historical factors that made German victory almost a certainty are present and accounted for, and those factors do lead to the historical result - the model is sound. This just divorces "who wins the game" from "who wins the war". Admittedly that might bug the heck out of the "I want to change the course of history" crowd.
2. Just One Punch
When I did the Table Battles expansion Age of Alexander, I had to grapple with the fact that he made everyone look like rank amateurs. He's doing quadratic equations in his head while they're struggling to count on their fingers. He's casually making a chocolate souffle from scratch while they're burning oatmeal cookies from a mix. They're not even remotely in the same weight class, and so time after time, our boy from Macedon routed entire armies while suffering minimal casualties himself.
The solution was to force the Alexander player to duplicate those results - they needed to achieve a complete victory while minimizing their own losses, with zero margin for error. Often the routing of just one Macedonian formation would result in the enemy player winning the match.
The advantage here is that it introduces meaningful challenges for both sides. Alexander has a lot to accomplish and a lot of tools with which to tackle that to-do list, but also has a lot to worry about - that player has to be very careful in how they use those tools. The opposing player only has one thing on their list - just get one of those cards to rout - but their tools are limited and fairly rubbish; unless Alexander flubs something, they really need to work to pull it off. But they don't have to worry quite so much about what's happening to this formation or that one (other than how it diminishes the tools at their disposal).
Like in our first method, this is acknowledging that one side is definitely going to win the war (or in this case, the battle) and that the other side is just trying to win a "game victory" by doing less badly than their counterpart. The disadvantage here is that while it gives our champeen roughly historical goals - deliver consecutive knockouts while keeping your guard up - it gives our pitiful palooka an ahistorical one: just land one punch. If the Persians had managed to bloody Alexander's nose a little, it's not like he would have thrown up his hands and hightailed it back to Pella. As a result, this is a method that I think works really well for something like Table Battles - a quick and somewhat abstract filler game - but would probably be less satisfactory in a more detailed and simulation-minded game.
3. Turning the Dials
Medieval and ancient battles often have wildly lopsided casualties, with the winning side suffering very few and the losing side suffering bunches and bunches, and the reason for this is that almost all of those casualties happen while the losing side is trying to run away.
So, there's a question you might want to ask yourself - was the thing lopsided because of factors X, Y, and Z, or did the thing become lopsided because one side started losing and the other started winning? If it's the latter, it almost always makes sense for the game to end at that point, before the slaughter begins. And that's generally what I do when modeling those sorts of battles.
In the Shields & Swords II and S & S Ancients series, victory is determined by victory points which are typically scored for eliminating enemy units. I generally start by adding up the VP values of all the units for each side, and I set the opposing victory condition to some percentage of that total, with that percentage also being determined by the desired length. Quick skirmish? I'm going to go as low as 20%. If it was fought from sun-up to sun-down, with great slaughter on both sides? I might go as high as 50%.
If the two sides are roughly symmetrical and evenly-matched prior to the decisive moment - if the thing was pretty much a toss-up until suddenly it wasn't - both sides will have the same percentage. If the factors I'm modeling make it easier for one side to get victory points than the other, then it's simply a matter of raising and lowering each side's thresholds until I get something that feels right. Maybe the disadvantaged side's threshold is at thirty percent, while the other side has to hit thirty-eight or forty.
But there's only so far you want to take this kind of thing. Asking one side to hit twenty-five percent and the other to hit fifty for example is in most cases pretty close to our "we'll call it a win if you can land a single punch" solution that would feel less appropriate outside of a short filler game. There might be exceptions of course. If there's a huge disparity in the total VP values of each side's forces, either because of the quality or the quantity of the units involved, then it's going to be appropriate to widen the gap more dramatically.
To a certain degree, this amounts to fudging it - to shifting the goal posts so as to artificially inflate the losing side's chances of winning. That's why I think it's probably most appropriate when modeling the sort of "roughly balanced until it isn't" conflicts I mentioned a few paragraphs ago, or when the disparity between the two sides is less pronounced.
Touching very briefly on the "balanced up until the tipping point" thing, one thing you'll find in the Shields & Swords games especially is that the tipping point tends not to be fixed. A side doesn't need to just hit 20 VP; they need to hit at least 20 VP and X number of VP more than the opponent - they need to get far enough ahead that the enemy troops despair of ever catching up. One reason why I much prefer the ancients iteration to the medieval one is that the rout mechanic, and bonus points for routing entire wings, really deliver on this "tipping point" concept.
4. Bonus Method: Throw Up Your Hands
This isn't one that I've actually used myself, but one I've always been fascinated by, and that's just to say, "You know what, the blue side just isn't going to win. Here's the game, have fun."
That's not to say that the blue side can't win. There's a victory condition for blue, and it represents what blue would have to accomplish to actually win the battle or the war. This is probably like asking blue to construct a full-scale replica of the Eiffel Tower out of pine needles. I mean, theoretically, it's possible, it's just not very likely.
Brian Train did something like this in his excellent game Summer Lightning, about the 1939 invasion of Poland. In practice, the best that a good Polish player can hope for is a draw. The rulebook itself says that an actual Polish Victory is unlikely unless using a number of the more extreme optional rules. Amusingly, after establishing that achieving an actual victory is far-fetched, Brian emphasizes the point by describing the ludicrous fever-dream ramifications of a Polish win: Hitler is overthrown, Kaiser Wilhelm returns from exile, and a three-way civil war erupts as the western allies steamroll into Germany!The thing is, just because Poland, like black, is playing for a draw, doesn't mean that the game isn't compelling or worthwhile. The Polish position might be desperate and even hopeless, but there are opportunities for stubborn resistance. And while the German position is strong, it's not an invincible juggernaut (historically they suffered over fifty thousand casualties), and the player must grapple with the defects of early blitz doctrine. The game makes cogent arguments about the campaign, and I'm not sure if those arguments would be as strong if the game's scales had been artificially tipped in Poland's favor.