As you can tell from the last post - I’m working on an upcoming war in my game world (Fletnern). There is a good chance that if a major battle breaks out, it could be “adventurers” vs. soldiers. So what? It matters. The adventurers will likely have seen more action than the soldiers, and will therefore be assumed to be more “veteran” and have better skills. But they also need to be skilled at a whole number of things, where soldiers can be a little more focused on their martial skills and still do OK. Therefore skill-wise, they are likely equal.
Adventurers often have magical weapons. That should give them an advantage. But on Fletnern, some of the troops do supply some of their soldiers with magical stuff too. Still, slight advantage to the adventurers.
Most of the soldiers will be doing this because they have some patriotism, whereas the adventurers may have opinions, but will likely see it more as a job, a job they can get just as easily in another town. Fierce loyalty vs. mercenary greed. Advantage Soldiers.
Here’s the big one, and one that will howled at by the various players (and maybe you too): Soldiers drill in tight formations. Adventurers don’t. Adventurers often use long swords and battle axes, whereas soldiers will be tightly packed with spear and shield. In following commands, in moving as a unit, as staying within the boundaries of the camp (and not getting picked off), in not getting in front of the firing archers - the soldiers have a definite advantage. An advantage so great, that it should be able to wipe out the adventurers.
But wait - look at most FRPGs and you’ll see that “men-at-arms” are low level and lack any real power. It is the adventurers that should rule the day. Well, if that’s what you’re FRPG says, you need a new game! I’ve already admitted that the more experienced adventurers would have more skills than the soldiers, and might even be better weapon’s masters than the soldiers, but the efficient fighting force that acts as a unit will be vastly superior to a ramble of skilled swordsmen running all over the battlefield getting in each others’ way. Sure, I’m exaggerating a bit, but let’s think about a modern example. African/Arab pirates against a modern naval vessel. Yeah - fire a grenade and run! Is that what this fantasy era battle will look like? Could be!