I always thought of war between Equestria and Earth, and who would win, would hinge on who would best be able to utilize the best of their qualities.
On Equestria's side we do have the obvious magic, but we also have a widely huge array different creatures and abilities, as well as relatively low technology, which is in part due to magic. Equestria would be reliant on a small part of their military having degrees of magic ranging from simple telekinesis, to a few OP individuals who have the ability to manipulate celestial objects (though how those mechanisms work, and if they could use those powers on the Earths sun/moon, we don't know). You also have to deal with most of those creatures very innocent, near pacifistic mannerisms, though in one alternate future we have seen them attempt as close to a total war as they could against the enslaved crystal empire.
On the other side of the field we have Earth, which has no magic but technology that subplants, or even shows better capabilities than magic, as well as allowing some technologies to be wielded by the average soldier that would put them at a better advantage than even the average unicorn (wireless communication, firearms, body armor), as well as certain technologies completely foreign to Equestria, such as atomic weaponry, Jet Fighters, Modern Logistics and Communication. Humans also are at a double edged sword at having fought within each other for so long; they are experienced at fighting and have learned the hard way what tactics work and don't, but at the same time being constantly against each other and forming alliances/pacts to actively or passively be against other groups would make it harder to work together collectively against Equestria.
Hopefully instead of that happening though, we can instead peacefully have ponies and humans live together, and maybe one day I can have a pony wife of my own.