Unlike our home planet, Equestria is filled to the brim with different beings who, if left unchecked, will not only endanger themselves but also the entire planet.

From the subterranean Diamond Dogs to the all eating parasprites.

Who or what do you believe could be used in a war against an army or nation, and of course how would you use them against said enemy(ies).

Taking advantage of the geographical position of Canterlot, I would use either Diamond Dogs or Quarry Eels to dig enough holes in that mountain so that the entire stability of the mountain itself would be compromised, making the entirety of Canterlot fall alongside the mountain.

I could also use Diamond Dogs to, obviously, dig under Ponyvilles Sweet Apple Acres in order to put only one parasprite down there. I'd obviously make sure that the roots of all those trees be exposed enough for the parasprite to eat in order to multiply. Within a day or so, the entirety of Sweet Apple Acres would be reduced into a desolate land as all of the roots of the trees (at first) would be devoured, then the trees themselves. And even if Pinkie could use all those instruments to guide them away, the damage would be done and it would take years in order for the Apple Family to recover.