Equestria is the greatest foe, the greatest country and the greatest victory that the humans will ever accomplish.
It will not be an easy conquest, because these foes do not fight with steel or powder, but with magic, and friendship.

And even after Equestria falls, even after the Princesses have been executed, there are still those who resist. Still those that fight back against their new human rulers.

There needs to be Captains, Officers, Lietenants and Soldiers who are willing to hunt down those who rebel, and put them down. These rebels are not all soldiers, but the elderly, the weak, the infirm and the children.
But they must be stopped, or else the humans will not be able to occupy their newly conqoured land.

Those ordered to wipe out these upstarts must have no qualms about executing ponies of all ages.
Of course, not every soldier is completely fine with following these orders. There are one or two who are quite the opposite…