Its true that Europeans used to fight each other much more. That's also true of most areas of the planet.
I think that for the main part you have Europeans working together now under NATO.
I believe that actually the most core reason for NATO is common genetic and ethnic heritage. In fact, to me the major global factions are essentially just ancient tribal disputes write large.
Now, this may make sense if you have a sort of 19th century Social Darwinistic pre-gentic-science eugenic worldview where human life has little value to you and each group is essentially an ant colony battling it out so that the greatest "subspecies" may procreate.
But we understand now that humans are humans. We have instantaneous global communications with increasingly effective translation systems. Our technology provides a relative abundance for much of the population.
Would it make sense for this paradigm of deadly force to apply to the people who live on your block? No, of course not. What about within your city? No. What about between the United States and Canada? It seems unlikely.
But if I bring up the example of the United States and Mexico? The idea that there might be mass killing to resolve conflicts would be much more palatable for many people unfortunately. And I think it comes down to ethnic and cultural differences. In other words, they have different family histories, look different, speak a different language.
Its very hard to actually see how much things are connected until things break down and the connections reveal themselves.
Applying deadly force in such a context is much more unpredictable than in the past. Even though we have enough examples of what happens (Read Niall Fergusons the Square and the Tower) the scale of interconnectedness and interdependencies as massive.
Think of the 2008 financial crisis, nobody cared about the banks, they even let two fail but the chain reaction it set off scared the shit out of everyone. Finance Ministers and Federal Banks in ever corner of the world across all kinds of difference were all of a sudden guaranteeing matching safegaurds and stimulus packages. From Russia to India to China. Same story will repeat with any kind military conflict between say US and Mexico, the actual interdepedencies will reveal themselves beyond the legacy reasons for grouping such as language, culture etc. It's a delusion that the connections can be severed. And sooner or later people learn that the hard way. It's good for health in the long run.