With all the commotion going on in the Ukraine, it can be hard to notice how minimal the chaos is in one respect — it is fighting among factions who want to gain control of a strong united nation, rather than one or more groups trying to violently break off. Turnover in leadership is one way to measure political instability, but an increase in the total number of nations in the region is an even stronger sign.
And it's not just in the Ukraine. Check out this map of separatist movements within EU nations, and this list of European separatist movements in general. There is strikingly little such agitation in Eastern Europe, and what breakaway groups there are would generally not result in an increase in the number of nations.
Rather, the group is culturally more similar to the people of a nearby nation than to the nation that they find themselves in right now. Perhaps they would like to join that nearby nation, and perhaps they want small-scale sovereignty. I think the more likely scenario is their territory being annexed into the nearby nation because asserting your common culture with an outside nation makes you think it only makes sense to join them in a Pan-Whoever-We-Are nation.
Call this "reconfiguration" rather than "fragmentation," then. Fragmentation is more when the breakaway group does not particularly identify with other groups nearby, and wants to be left by themselves.
The Ukraine actually has one of each type. Carpathian Ruthenia has a separatist movement that would naturally fit better with a Central European nation, and if successful would follow a path of reconfiguration. On the other hand, the Crimean Tatars don't have a nearby nation to align themselves with, and if successful would press for their own sovereignty and increase the number of nations.
Still, these cases are fairly marginal in Eastern Europe, whether measured by numbers of people who would be affected, the percent of the population that would change nations, or the area of land that is under dispute.
Look at Western Europe and the picture is totally different. Consider the southern part of the West — all those regions in Spain don't want to join up with any other nations, as they consider themselves the furthest extent of Whoever-We-Are. Galicians, Catalans, Basques, etc. Then move up north to the British Isles — Scotland wants to be by itself, Ireland has already split off unto itself, and there's even an active separatist movement within the tiny little nation of Ireland, seeking to split Ulster off from the rest.
The example of Northern Ireland serves as a reminder that there is no natural lower-bound to how small the splinter group can get. It's not as though Belarus has no such groups because it's already rather small and could not splinter any further after the breakdown of the Soviet Union. It sure could, but something is keeping the Slavic country together in a way that is lacking in the Celtic country.
One explanation for the greater scale of national cohesion in Eastern Europe would be Peter Turchin's model of ethnogenesis, whereby national solidarity is intensified along a meta-ethnic fault-line. That's where the groups on either side are so different from each other — different mode of subsistence, language, religion, race, clothing, etc. It is especially strengthened when one of those sides exerts power and influence over the other for so long. Over time, that binds the pushed-around group together, while the formerly glorious nation comes to rest on its laurels, becomes divided internally, and is weak to the attacks of the now united and motivated group on its periphery.
In this case, Eastern Europe would be glued together by the expansion of the Ottoman Empire during the middle part of the second millennium. True, the Turks are not the terrifying force they used to be, and they've been all but driven out of the Balkans. But national solidarity is slow to fade, as is the power and influence of the expanding nation that is now contracting. And before the Turks, there were the Mongols, and before them all the other waves of Steppe nomads.
At least compared to Western Europe, which has seen no such intense pressure as a whole region since the Roman and Holy Roman Empires, Eastern Europe must have a greater residue of solidarity, albeit at a decaying rate since the Steppe invaders are no longer pressing at the gates.
I think that could work for the parts of the East that were strongly subjected to such outside pressures. But Poland, the Czech Republic, Lithuania, and Finland did not bear the brunt of the Ottoman invasions, yet they are still remarkably well held together — unlike Spain, where Arab and later North African Muslims not only invaded but ruled the peninsula until they were finally driven out in the 15th century.
Something additional is gluing Eastern Europeans together more than Western Europeans.
My hunch is that it's the greater degree of genetic relatedness among Easterners. A recent study looked at how much genetic variation is "identical by descent" among various European groups. That is, they are genetically similar not because they evolved independently yet similarly, but because they descend from a common ancestor population. Since that splitting-off period, they have accumulated their own distinctive genetic variants. But descending from that common group gives them a good deal that's shared too.
Which part of Europe do you think looks the most similar genetically? Yep, it's Eastern Europe, from Poland down into the Balkans, even including Greece (though to a lesser extent). Folks in Spain or Italy have greater genetic distinctiveness within their own countries, forget about across countries. Just because they both speak Romance languages, were both part of the Roman Empire, and enjoy great Mediterranean weather, doesn't mean they share much blood with each other. Northwestern Europe isn't quite as patchy as Southern Europe, but it is still not as uniform as Eastern Europe.
What accounts for the greater similarity in the East is the recent, massive migrations of the Slavs — or if you want to call the source population something more pedantic, imagine I'm writing Slavs in quote marks. Those migrations began in the second half of the first millennium, more recently than any other mass migration in Europe aside from the Germanic ones from the mid-to-late first millennium. The Germanic groups, however, found people already settled in the lands that they invaded. Unless they drove them out en masse, they only ruled over the non-Germanic groups but did not contribute much to the local genepool. Germanic groups ruled Spain in the middle of the first millennium, for example, but didn't leave more than a drop in the bucket genetically.
The Slavs fanned out toward the north and east of Europe, where large sedentary civilizations were non-existent. We know that the Baltic languages used to be much more widespread in Northeastern Europe, so the Slavs must have displaced / killed their cousins the Balts when they took over what is now Russia and Poland. But the Balts were not a long-standing civilization that was difficult to dislodge, as the patchwork of groups in the Mediterranean were for the Germanic invaders.
Northeastern Europe is also very flat, being part of the Great European Plain, and it's easier to drive out people on a flat terrain. It's harder when they command the high ground in more hilly / mountainous areas. That's one thing that stopped the Slavs from totally taking over the Balkans, a largely mountainous region where the Albanians and Greeks are still hanging out, each of them being a sui generis branch of the Indo-European family. That must have been even stronger in Western Europe, where the Alps, Pyrenees, and other lesser ranges have allowed the pre-Germanic folks to hang around — some kind of Celtic and Roman mixture for the French-speaking areas, and the pre-Indo-European Basques.
In a sense, then, the Russians, Poles, and Bulgarians are not like the Portuguese, Spanish, and Italians, or the Irish, Dutch, and Swedish. It would not be too great of an exaggeration — especially in the comparative context — to speak of Slavs who have settled Russia, Slavs who have settled Poland, and Slavs who have settled Bulgaria.
Pan-Slavism nearly came to fruition just in the past century: the Soviet Union brought together the Western and Eastern groups, and Yugoslavia brought together the Southern groups. For a time, it looked like they might have merged, but the plains Soviets and mountainous Yugoslavs were just a bit too different for that. Bulgaria remained technically outside of either, but true to its lowland geography was more a satellite of the USSR than of Yugoslavia.
Both of those super-regional nations have broken up within the past 25 years, but don't count them out just yet. The high degree of genetic similarity gives them less of a barrier to clear in considering one another as brother-countries rather than mere neighbor-countries.
Also bear in mind how impossible this has proven in Western Europe, at least since the Holy Roman Empire of the early second millennium. You won't even get the British Isles to unite, forget about Pan-Germanism. The Nazis offered, and nobody took them up on it. Ditto for Southwestern Europe — Italians can't stand being governed along with their countrymen from too far away, and the same is true within Spain. Those two haven't been united under a strong nation since the Romans.
In that episode of Seinfeld where Kramer and Newman are playing Risk, it's not a Spaniard who objects to their game on the subway, saying "I from Spain, you not say Spain is weak." It's a Ukrainian who bellows out his defense of the fatherland.
The only thing that could show up the Slavs is if another group underwent a mass migration, displacing locals and swelling in numbers. They would be even more related than the Slavs. By this point, though, Europe is filled up. The Slavs were the last to enjoy a mass migration into not-so-occupied territory. Garden variety immigration will not do the trick, since they are drawn from too many sources. It would have to be a small initial group who steamrolled over everyone else, and left descendants from their own smallish group. And the opportunity for that is gone.
The meta-ethnic fault-line is well established as a potential force for gluing a people together into a nation. But genetic relatedness needs to be taken into account as well, especially when the group is not just some small clan of highlanders but nearly half a continent.