Other users have discussed actual combat, but I'd like to mention some points about logistics and broader tactics
Topography and geography would be a bigger deal
In general, the rule of thumb is that the smaller you get, the more uneven the landscape becomes from your perspective and the more difficult it is to get around anywhere. As an example of this, consider a river. For a an army of human-sized beings a river is easily fordable or it may even be possible to wade across, whereas for your miniature humanoids even a small river might be unfordable for a standing army and would require specialized bridges or boats to cross. Figuring out how to get from point A to point B as well as avoiding rough terrain to keep an army from getting boxed in would be a big deal.
Similarly, because a landscape is more complex at a smaller size, this opens the door for new tactics that wouldn't be possible for humans. A good example of this would be setting up an ambush where one side rappels down from the trees. A large number of humans could not easily hide in a tree unless that tree were very, very large, but to a 12-inch-tall being this kind of ambush could be set up in almost any forest.
Local wildlife is a bigger deal
As you get smaller, the number of potential predators or other dangerous wildlife dramatically increases. Additionally, while humans have pretty much wiped out anything large enough to cause a threat to us (aside from other humans), this would be a lot harder at smaller scales because dangerous megafauna can more easily hide and survive.
For a second, let us compare a human army with an army of ants. A human army is large enough that any tiger or elephant is liable to get out of their way, and it would be very unlikely that a standing army would have anything to fear from either. A column of army ants, on the other hand, has issues where workers and soldiers get picked off by other predators such as mantises. Yes, in the link shown the ants eventually won, but at the same time they had to deviate from their plan and waste time and troops fighting and killing a much larger predator. Imagine a world where a standing army has to worry about one animal having a bad day and deciding to ruin all of their pre-laid plans by taking out half their army. Imagine a world where instead of just having to worry about one enemy forcing marching around the landscape, armies have to deal with constant guerilla warfare fighting off predators that have no allegiance, no organization, and no supply chain. At least with another army you have a vague idea of where they are.
For a fictional example, check out Amphibia. Despite the fighting between the main factions of amphibians, half of the threats in the series come from the local wildlife as much as other sophonts. Indeed, two plot points early in the series come from a standing army being wiped out by a pair of herons and the largest, most-developed city in the world being under siege by a bunch of non-sentient ants.
Supply lines might be more vulnerable
In general, smaller animals have higher metabolisms than larger ones. An elephant eats more food than a mouse, but if you have a number of mice equal in weight to an elephant, they will actually eat more than the elephant. This is especially pertinent for smaller endotherms as the higher surface area means they lose heat faster and have to eat more to maintain their body temperature. Terrestrial sophonts pretty much have to be endothermic in order to keep their brain fed and warm. Consider the most intelligence modern animals: primates, cetaceans, elephants, pigs, crows, parrots, and octopodes. Of those only octopodes are really cold-blooded, and they're in the water.
This means your small humanoids will require more food than a human army of similar mass, and hence will be very vulnerable to starving if their supply lines are disrupted. In fact, they might also be more vulnerable to death by exposure in desert or winter conditions as they have a harder time staying warm or cold.
Another major hurdle is fire. Fire requires the same amount of fuel no matter what size. This is one reason it is thought that ants don't use fire, it consumes more fuel than the ants can feasibly gather to keep it going and can easily cook them if they aren't careful. Your species might be big enough to use fire, but they would have to carry around proportionally more fuel than humans would to keep the fires going, which means they would be