GURPS Mass Combat is a reasonably comprehensive set of rules for running large scale battles in GURPS. It has rules for different types of troops with different levels of training and equipment, for raising new units and maintaining existing ones, and for fighting all different types of battles. One thing that is mostly missing are rules for improving the quality of existing troops as they gain battlefield experience.
There are two references to improving units: on 12, under Improvements, "The GM may allow troops to improve in quality... if events justify it," and on p 14, under Disbanding Elements, "For flavor, an element may be converted rather than disbanded." There's no real guidance for deciding when troops should improve in quality, and so I'm writing some house rules for New Dawn to provide some guidance.
Each time a unit survives a battlefield, it gains 1 experience. It then loses 1 experience if it took more than 40% casualties, half its experience (round the remaining experience down) if it took more than 50% casualties, and all its experience if took more than 75% casualties.
A unit can be retrained to lock in its experience and improve its troop quality by 1 step after earning 2 experience. Retraining takes one week per $10K of the difference in the raise costs of the improved troop and the unimproved troop. Units with more experience are cheaper to retrain: each extra point of experience reduces the cost by 20%, to a minimum of 0. The retraining time is based on the adjust cost, but can't be reduced below the lower of four weeks or the normal time to raise the unit. A unit cannot move or fight in any battles while it is being retrained, and if it moves or fights in a battle, an entire week of training needs to be redone.
A very experienced unit can improve its quality by multiple steps at once instead of reducing the cost of training. It takes 8 experience to raise the quality by two steps or 15 to raise it by three.
Instructors, Cadres, and Replacement TroopsTroop quality is determined by training and culture and all new troops are raised at the appropriate quality level. If an army has created higher quality troops through battlefield experience, it may not be able to raise fresh units of the same quality to act as replacements. Adding lesser quality troops to a high quality unit generally lowers the quality of the high quality unit, but there are ways to get around it.
In general, if a unit gets has its size increased by more than 10% and the new troops are of a lower quality than the unit, the unit's quality drops to the average of the two quality levels: adding Average reinforcements to an Elite unit turns the entire unit into a Good unit. Any battle experience is lost.
CadresA unit can absorb more reinforcements if it is given time to absorb them and train them to the unit's standards. A unit can take up to 60% of its size in lower quality reinforcements without losing quality (and indeed, raising the quality of the reinforcements) by taking four weeks to train with them. This training is expensive (pay the cost to raise the quality of the reinforcements normally) but is an easy way to quickly produce a high quality armor. The high quality unit loses all its battle experience and can't absorb any more reinforcements without losing quality until it has gained at least one battle experience.
Units acting as cadres and absorbing reinforcements cannot move or fight in any battles and must repeat the full week of training if they do move or fight in a battle. The reinforcements are keep their lower quality until the training with the cadre unit is complete.
InstructorsA high quality unit can be used as instructors to improve the quality of newly raised troops. Each element assigned to act as an instructor can train ten elements to its quality. Raising new elements with instructors has the normal cost of raising a new element of the appropriate quality. Instructors don't save money, they just allow veteran units to raise better units than a nation's normal training programs.
Elements acting as instructors cannot move or fight in any battles and must repeat the full week of training if they do move or fight in a battle. Elements in the process of being raised do not exist for mass combat purposes until their training is complete.