Actually the foreign army thing is relative to something that is happening to us right now- the fall in the birth rate of the native dominant class. With the Romans, it was probably the introduction in the widespread use of lead pipe around 200 AD or so that caused a drop in fertility (and smarts). Since bathrooms and such was the high tech stuff of its day, basically the Roman citizenry was affected and not the slaves or foreigners. The low birthrates forced them to need both more slaves and foreign mercenaries to help them get them. The same thing is happening here in a more modern way, except it is being caused by birth control and abortion among whites and blacks. Latinos, our modern wage slaves, do not practice either method widely due to their more orthodox adherence to Catholicism, as well as cultural factors that encourage them to have large families. No telling where that will lead, but you can pretty much predict that America and Mexico will unite as a single nation over the next century.