You could do a PhD thesis on this subject, but unchecked European nationalism would be the most obvious common thread that covers all that. European nationalism is the toxic stew that has started every war and incited Europeans to slaughter each other for the past 1000 years. Take World War I for instance. No one has ever successfully explained how WWI started in terms of clear-headed rational thinking. How does the assassination of a single Austrian Archduke in the streets of Sarajevo justify every country in Europe slaughtering millions of their fellow Europeans over the next 4 years? The textbook answer is there was a system of treaties and alliances in place, but why on earth are millions of Germans trying to kill millions of Frenchmen and Englishmen because of a lone Austrian that nobody particularly cared too much about? It makes no sense, until you look at the unchecked rampant nationalism. There was national pride, ego, honor, and unchecked militarism that was spoiling for a fight involved. There was a way too eager rush to war and a desire to use muscle and especially the new weapons of the 20th century; the airplane, the machine gun, and new super big cannons.
After World War II the predominant and still current thinking among heads of state is that the best way to prevent any future European wars is to dial down individual country nationalism, but to dial up the idea of pan-European nationalism. Hence we now have treaties that certain scarce resources belong to and can be purchased by ALL of Europe. Nobody can cut anybody else off from a needed resource or use it as a bargaining chip. European leaders then worked towards creation of a European Union (check), and then finally a common European currency (check), and so everyone's fate is now pretty well tied together, financially, logistically, militarily and most importantly, emotionally. So far, it has been working pretty well preventing any European wars for the past 75 years.
