Eli5: How do they decide the boundaries between nations?
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What we now call Europe was mostly drawn by political wrangling after World War II.
In the 1700s and 1800s the continent of Europe was largely controlled by just a few dozen wealthy barons and emperors (eg. Austro-Hungarian empire), and they spent a lot of time in “little wars” either settling disputes, reclaiming territory that had been taken earlier by someone else, grabbing this river access or that port because they felt they deserved it, etc; they were almost all related (royalty can’t marry commoners, so you would marry royalty from another country if your own country didn’t have anyone eligible).
WWI happened, in short, because Serbia was controlled by Austro-Hungary, but the Russian Empire felt protective of Serbs and other people groups in eastern Europe on the belief that they were all technically Russians (a claim which Putin perpetuates, by the way). The heir to the throne of Austro-Hungary was assassinated while visiting Serbia, and Austro-Hungary blamed the ‘kingdom’ of Serbia and decided a “little war” would be a good way to remind Serbia of who was boss. How much backing/knowledge the Serbian government had of the splinter group’s assassination scheme is still debated, but that’s an aside. They blamed the country as a whole. Anyway, Russia being friendly to the Serbs at that point said “oh hell no”. Once Russia was distracted with this “little war”, Germany decided this was probably a good time to settle a long-simmering border dispute with France. France and Russia were friends at that time, and if Russia was tied up in eastern Europe maybe Germany could stand a chance of settling their dispute with France. But Germany made the mistake of marching through Belgium in order to get to France (despite Belgium saying “no”), and Belgium was in a protective treaty with Britain.
And things continued to spiral from there with Italy, Turkey (in the form of the Ottoman empire), and eventually the US getting dragged in. By the early 1900s these empires (Germany, Austro-Hungary, Prussia, Russia, Ottomans, Napolean, etc) had spent 200+ years moving and relocating peoples and using “little wars” to shift borders every few years. This is part of why you end up with German speakers way over in Lithuania, for instance. Keep in mind that several of these empires had overseas colonies & holdings, and in one for-instance Japan decided that all these powers were distracted in Europe meant this was the perfect time to do hostile takeovers of the European claims of Pacific territories, and to go after bits of China and eastern Russia. The whole world literally did the “[pointing Spiderman](https://i.imgflip.com/55f137.jpg)” meme, except with war.
WWII was Hitler taking advantage of the terms of the treaty that ended WWI to rile up Germany, but that’s another story.
After WWII the political leaders in Europe basically sat down together and decided Great Wars were not sustainable and that little wars like the 1800s empire version were a great way to end up with Great Wars. So they did something else — they settled boundaries politically because there was no way to draw lines that included (or excluded) any one language, religion, culture, etc. Rivers and mountains are one type of easy boundary, and disputed areas were negotiated. And perhaps most importantly, they set up the concept of frictionless trade and travel so that if a German speaking person in northern Italy wanted to visit their cousins in Germany, they could do so with nothing more difficult than showing their license at the border (if that). You could also work or sell across borders. This made it so that political leaders could do things like build roads or operate courts in their country without having to worry about negotiating hundreds of treaties for each and every group of language or religion that was split by a border, and thus reduced the social-political tensions that had existed previously under micro-managing emperors. They agreed to common carrier standards for things like railroads and measurement systems. (On the note of Railroads, Ukraine’s railroads are still the Soviet standard, which is why grain in Ukraine can’t simply be put on trains and shipped to Poland or Romania for sale; the Soviet and European rail gauges are incompatible). This is also [why east and west Berlin](https://lossofthenight.blogspot.com/2013/08/first-blog-post-welcome-to-official.html) can still be distinguished from the air at night based on the streetlights, and so on.
Basically, the kings and ministers in Europe collectively agreed to just make a border -any border- for administrative and local economic purposes while simultaneously ‘ignoring’ borders for social purposes and macro-economics. This intention is where the idea of the EU came from. The attempt is to retain internal political control (eg. your own military, your own taxes, etc) while reducing or eliminating as many international stresses as possible (compatible rail systems, common currencies, etc).
But this is relatively unusual in the long arc of history, the only quasi-similar comparisons in history might be the Persian empire, the two kingdoms of Israel (between themselves), and the Iroquois confederation. For the most part, if two nations were powerful and one was warlike the boundaries would be settled by war or intimidation. Or if there was either parity between the nations in terms of military or if neither is warlike, they might settle on a natural boundary such as a river, a row of hills, or other natural features. Or a combination of the two — perhaps most famously, when Rome invaded Britain in the first century they pushed further and further north until they got to the area that is now Scotland (roughly) when a combination of the native people and the “wilder” terrain halted their advance. At that point, Rome set up the boundary along a naturally existing series of cliffs and dikes. They set up forts and patrolled the boundary, eventually adding wall elements to enhance the natural barrier (see [Hadrian’s Wall](https://en.wikipedia.org/wiki/Hadrian%27s_Wall)). The wall was ultimately unsuccesful but it did help a bit in periods where it could be fully manned. (Ditto the Great Wall of China, walls in Mesopotamia, etc).
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