Mount & Blade: Warband is a classic game that defined, and may well have created, its genre. But like all games with historical connotations, inspirations were taken from real-life figures and countries.
Starting off with the most basic of factions, the Swadians, one would argue that they are not as basic as they seem. Drunk on the power of past empires, they try to live up to their name as successors through warfare, and overindulgence. It is evident that Swadians are inspired by countries such as France after Charlemagne, or the Holy Roman Empire, both of which were successors to Rome, at least in military might, if not cohesion or anything else really.
Alongside the Swadians, but definitely not in a friendly way, are the Rhodoks. Being culturally similar due to their relatively shared roots, Rhodoks may be equated to Venetians due to their prevalent usage of bows and heavy armor, geographical location filled with hills, and sea-based trade routes along the coastline of the Calradian, that is the Mediterranean, sea, and are the successors of Romans as much as the rest of them, though there is only ever one true successor, isn't there.
The Nords on the other hand need no introduction. Embracing the spirit, culture, and even the Nordic name, they embody every stereotype about Vikings in existence. They raid and pillage, fight exclusively on foot, and live to die in combat.
Vaegirs, just like the Nords, embody the stereotype of an ethnicity. That ethnicity being Slavic, of course. With their armor, both nomadic and settler lifestyle, as well as geographical location, that is in perpetual winter, Vaegirs are Slavs, through and through.
Both the Sarranids and the Khergits, are the Abbasid Sultanate and the White Horde, respectively. Following their more successful ancestors, Sarranids are no longer conquerors they used to be and struggle to hold on to the territory they already have, while the Khergits border the Vaegirs, in the same way, the White Horde settled near Russia, after the death of Genghis Khan.