[quote=@Willy Vereb]
BTW, I'd advise against underestimating what the fantasy side can do. Heck, what even numerical superiority in soldiers can do if utilized well. Advanced technology has a lot of benefits and not respecting it can lead to hilariously lopsided battles. On the other hand be it an arrow or a guided missile, they both can murder a person. Given the right conditions those silly mud dwellers with bows could handle your ass in warfare.

Take a stereotypical tree monster as an example. First off, no, neither gasoline nor diesel would be sufficient for this purpose. Gasoline would explode instead of burn in these conditions and diesel would barely ignite. Even if you choose the right accelerants a canisterful of them would only really work if you soak the leaves and branches and hope the crown of the tree generates enough heat to burn the trunk. Live tree hunks can be surprisingly difficult to burn. 
Okay, what about other measures? Explosives can work but again, only if you use enough of it. Explosive tree removal is a thing, especially in America. But how much explosives you require for it? Pounds of it to break a tree in half with carefully positioned charges. A single explosive may do severe damage to a treant but being animated tree with no vitals or sense of pain, that won't deter it.
What if you don't have much in explosives? Well, expect tens of thousands of rounds being spent on a tree each. Preferably from distance so the deflecting bullets won't injure your own men.
Long story short a treant is "easily" beatable by a modern army but you should never underestimate the amount of commitment this would require. Face an army of them and you probably run out of ammo way before you eliminate all of them.

<Snipped quote by Wildman13>My elvish empire would be a moderately sized nation where subjects of other races are ruled by militant elves. I think I'll base them mostly on my Yllendthyr NS from other fantasy games but not entirely.


[/quote]

Well in here the elves have the ability to make contructs. Something you might like more than guns.