combat without players

Let's talk about combat, shall we? Combat can be quite easily the most rules-heavy part of the game, no matter the edition involved. Those rules are pretty clear as to what is going on, and work decently well across the board. I am not one to fix what isn't broken, but there are some parts that, to me, are just a glaring omission. LS over at Papers And Pencils touched upon the subject with his fast-playing skirmish. There are no mention of skirmish combat rules. Not quite army against army, but large enough group that rolling dice for each and every combatant is completely out of the question, as it would take 3 hours.

So here's how I do it. This system was designed with GURPS in mind, to be used during a pirate game for boarding action, but it is generic enough that it should be able to be ported over to pretty much any system. It has been divided into two sections, unit vs. unit, and man vs. man. They both use the same basic formula, which is has follows: attackers and defenders both roll a single D6 and add their bonus. The two result and then compared to achieve a result.

Let's start with man vs. man. Obviously this rule would be a bit more rare then the unit vs. unit one, but it is a bit more simple, so i'll start with it. Also, this would only come to play when two NPC fight each other with no intervention from Players.

• First step is determining the toughness of both combatant, or how many hits each of them can take. This is fairly easy in level-based system like D&D, just use the combatant's level.
• Second, determine the modifiers for both combatant. Modifiers are obviously up to the GM, but some  that I found useful are the combat ability (full level for fighter and variant, 3/4 level for cleric and thieves, and 1/2 level for magic-users) and weapon and armor quality. Beside level, each modifiers should only give a +1 bonus or so, as to keep the numbers manageable
• Each side roll a roll a D6 and add their modifier. 
• Substract total from NPC B from the total of NPC A. If the result is positive, NPC A scored a hit and  1 point is deducted from NPC B. If the result is 0, Both NPC strike each other. If the result is negative, NPC B scored a hit. If both NPC roll a 1, neither score a hit.
• Repeat step 4 and 5 until one combatant has been reduced to zero toughness or give up. 

So the whole formula reads like this D6+(mods) - D6+(mods)

Now, unit vs. unit is a bit more complicated, but the basic steps are the same. The modifiers change a bit, such that you could give a +1 if one side is fighting with officers, + 1 if their morale score is over 100 (for AD&D style morale system), -1 if their morale is under 50, +1 if they are outnumbering their opponents, +2 if they outnumber their opponent 2 to 1, etc. Season to taste. The difference is that the final number is multiplied by 3 to achieve the number of casualty suffered by the loser. 

The formula thus read 3((D6+(mods)) - (D6+(mods)) = no. of casualty. 

Some other modifier, if the defenders are unable to strike back or surprised  treat their D6 roll as being equal to zero, but add the modifiers. 

Obviously, only one side will suffer casualty per round. This is okay for me, as the whole thing is abstracted. If this bothers you, you can simply double the rolls, and treat any result of zero or lower as no casualty.