Growing up in Belgium in the 1970's/80's I remember my grade school history class. In the General History Of Our Country department there were two 'really terrible' times. First was the trench war of 1914-18. It was considered more devastating then WWII because 1) it happened in our backyard and 2) unlike WWII, which was for Belgium just 3 weeks of getting clobbered and then 4 years of being occupied, there were actual Belgian troops getting killed in the trenches from the first to the last day. But that aside...
What I wanted to talk about was the second instance of life in Belgium being so terrible. That was the 80-years war, otherwise known as the religious wars, the Reformation wars or 'the 30-years war plus all the fighting that came before it"
In other countries this might just be covered as a prolonged war of independence on behalf of the Dutch with some sideshows by the Swedes and the Royal Navy sticking it to the Spanish Armada. In Belgium, especially in Flanders, it was two, almost three generations of constant civil war, ostensibly between Catholics and Protestants, but actually between constantly shifting bands of marauders more or less part of the Spanish or Oranje armies. And once the fighting had moved away, there was ethnic cleansing and persecution of real and imagined supporters of the previous occupying force. And of course there were burned fields, looted winter supplies, hunger and disease and random acts of rape and torture pretty constantly. And with it going on for nearly 80 years, there were people that had lived through this since childhood and never known things being different, even in the tales of their parents. So that when peace finally came it was a culture shock just as big as 'first contact' would be today.
Of course, this happened between 1560 and 1646 so it does not really fit the post-1900 label, but after witnessing the violence in former Yugoslavia, it isn't such a hard stretch to imagine a 80-years guerrilla war ripping Europe apart from the inside. May be a more violent Communist revolution spreading through Europe in the 1920's, may be victorious Nazi Germany bogged down in a guerrilla war in the East, that eventually spread to the western countries as well? May be a more virulent 1968 with ' Tupamaru'-style city guerillas fighting ever more dictatorial repressive regimes like in South America? May be even the Balcan conflict spreading out into the former Warshaw pact states and eventually the west as well?
Anyway, not sure how it may come about, but just even 15 years of incessant civil war or insurrection by guerilla forces would mess up every country pretty good. The 'modern' European states may be even more than a lot of other, 'second-rate' countries.
