Either Equestria went to Hell, or the Marines are in Purgatory. Either way, this isn't going to be good for either of the involved parties. To an extent.
Will the Humans be able to overcome their warrior instincts and befriend this seemingly perfect paradise? Will they be able to stand each other and themselves, or will conflict on ideals and morals intervene. Will they resume their singular squadron mind or discover themselves? Or will they let their free-will and baser instincts take over, resulting in their Assault on these strange things? Will the Equestrians be able to handle these Beasts of Damnation, or suffer the underserved wrath of mechanical warfare?

NOT a Chrysis crossover! It'd have 'Crossover' on it. Just know what not to expect.