I just wanna see Ponies Vs. Nazis Vs. Mongols Vs. Vietnam Era Americans.
Couple observations:
* It wouldn't take a very clever unicorn to levitate an ursa into an invading army, drop it on its head and simply leave.
* In Boast Busters, the water tower that twilight telekinetically ripped from the ground was outside of town. There's no line of sight limitation on unicorn magic. She was able to pull a fairly massive object containing about 15 tons of liquid (the math was uninteresting, but that was my estimate), perform fine motor control by milking cows to fill it, then float the whole thing into town, and give it to the ursa. And then pick him up too and levitate them both clear out to a cave in the Everfree forest. Even without computing the weight of the ursa (difficult, as it didn't appear to be flesh) that's still an impressive feat of magic. And Twilight is basically a college student. Talented perhaps, but a "college students" in Equestria are telekinetically lifting 15+ ton objects while performing fine motor control with them and quickly moving them across perhaps miles of distance without line of sight. How many much older, more experienced unicorns are there also able to perform feats of similar magnitude?
* We don't have much information on the full extent of Celestia and Luna's power. We know they can move celestial bodies, but does that mean that Celestia is telekinetically accelerating 10^27 tons of mass at 93 million miles distance fast enough for it to be clearly visible as several degrees of motion? If that's the kind of magnitude of force she's able to exert, then it's
totally reasonable to suggest that she could levitate an entire army and telekinetically crush it into a super dense ball of goo, possibly without ever bothering to leave her palace. But we have to make all kinds of assumptions, that the mass and distance of Equestria's suns are similar...that they're even stellar phenomenon like we have in our universe, etc.
Since it appears to not take several minutes between Celestia raising the sun and its light being visible, the Equestrian sun is presumably much closer and smaller than ours. And moving the sun and the moon can be interpreted in ways other than direct telekinetic force. The Equestrian sun might not be a sun like in our universe. Banishing "Luna" to "the moon" might have been a non-physical event. When Celestia appeared over Ponyvile in Lesson Zero she might have flown rather than teleported. Some of her demonstrated feats can be explained away. However...
* In Friendship Is Magic part 2, Celestia does a very interesting thing: she verbally responds to Applejack, and
then flies from beyond the horizon as a ball of light to a compete stop inside the room over the course of about 8 seconds.
Couple things:
1) She is very visibly a ball of light, and not in pony form. Presumably that would take care of being liquified by acceleration forces. But if she's able to transform into a non-corporeal light body, then it's reasonable to suggest that she may be able to render herself immune to any kind of mundane weaponry. What effect will swords and artillery shells have on a ball of light? Additionally, since she's able to casually deconstruct and reconstruct her body from physical pony to light, it's plausible to suggest that destroying her physical body might not kill her. She might be able to simply make a new one.
2) She was aware of the conversation in the room and was able to project her voice into even though she herself was not there. This is consistent with Twilight's feat of lifting the water tower from outside of town. Unicorn magic can clearly be used to casually observe out of sight locations from miles away and perform magic remotely at that distance.
3) Since Luna was banished to the moon, it's reasonable to suggest that Celestia might have been banished to the sun. That she appeared as a ball of light from beyond the horizon would tend to support that interpretation. If so...the implication is that she only not only did all this from "a couple miles away" but rather, she may have done all of this from an
interplanetary distance. If Celestia is to observe events while not even on the planet, use magic to locally create the sound of her voice at that distance, then engage in planetary reentry and near-instant deceleration, all while not even physically manifest in her body, and from the calmness of her voice, and all of this with minimal effort. Some of these feats might not directly translate well into combat capability. But the general level of magical competence and power she's demonstrated tends to support the interpretation of Celestia as a reality-warping
Goddess rather than merely a magic-wielding unicorn.
3a) If she was banished to the sun like Luna was banished the moon, and if you interpret that as a physical event, that implies that she's able to withstand temperatures and gravitational forces present on a star, lack of oxygen, etc. And if you don't interpret it as a physical event, it once again suggests that Celestia's physical body is just a convenient and temporary shell she's wearing. Destroying it is unlikely to harm her.
4) If they do manage to kill her, life on the planet basically ends because the sun won't rise or set anymore.
I don't think human armies, modern of otherwise, stand much of a chance.