Have ever even seen a cover of a history book, in a bookstore, across the street? Because what you say is so contrary to history I don't even know how to start...
Back in the day there was pretty much no such thing as nationality, only ethnic and cultural differences, but those in no way stood in the way of recruiting levies. Sure, the population of a different culture could be more reluctant to obey, but in the end, unless they could revolt, they would join whatever lord currently rules their area to war because that's their obligation. And if they don't fulfill that obligation, bad stuff happens.
A regular peasant wouldnt even know what "country" they fight in, they probably wouldnt necessarily even know who their lord is at present. All they needed to know is that a recruiter visited their village and its time to grab a spear and shield and go. (or pay some money to avoid going, if they can)
Also, there's the fact that it was a very common practice to recruit troops from particular regions and ethnicity for their specialties, like Balearic slingers and Cretan archers in the ancient times, and genoese crossbowmen in the middle ages. I mean, the Varangian Guard of the Byzantine emperors was composed solely of Norse, and later Norman warriors.