To give an example, in the Netherlands you have the lowe Rhine branches, which is a river that flows from Switzerland all the way into the North Sea. This area was one of the natural borders of the Roman Empire in the antiquity and there is somewhat of a cultural divide amongst modern people living on both sides of the river.
Before the Romans visited these swampy lands there was already a "split" so to say, on the northern side you had Germanic people and on the southern side you had Celtic people.
Obviously this was not a clean divide as Gaul and Germania were not two nation states but areas inhabited by Celtic and Germanic tribes. If you were standing in the middle of the border the people north and south would seem really similar but the more north you go the more Germanic they become and vice versa for Celtic Influence in the South.
There is some genetic evidence that there was a divide because even nowadays there is a bit of a split in Dutch genetics, people with more southern ancestry drift more towards central Europeans (Celts are as Central European as can get) while northern Dutch people are a bit closer to Scandinavians.
But yeah back to history, Caesar helps out the poor Gauls in their fights with Germanic barbarians by invading Gallic lands, kill like 1/3 of the population and enslave another third. Because of big Baller Caesar the southern part of the Netherlands became a border region of the Roman empire while the area above it was part of Germania, the endless forestlands and swamps were crazy giant marauders lived.
So for a few hundred years the Roman empire survived and thrived and had a great influence on the gallo-germanic inhabitants of their border region. Rome became Christian and so did the other areas, while the realms above the border remained fiercely pagan.
Fast forwarding a bit: The Roman Empire collapses, the Franks rise to power and conquer the province formerly known as Gaul, the Franks convert, Charlemagne goes on a jihad against the pagan Frisian and Saxons and stamps out paganism in the Netherlands. Now the entire country is Christian, sort of.
Another fastforward unto the middle ages and you have this guy Martin Luther who nails a piece of paper against a church and essentially starts a new wave of Christianity. Which areas seem to really pick up on this new wave called protestantism? The areas that were beyond the borders of the Roman Empire! And the areas that remained predominantly Catholic? You guessed it, Roman territories.
Obviously history is far more complicated than that because you also have other big reasons for that Catholic/Protestant divide in the Netherlands, such as our revolutionary 80-years war for example, but I think even that in a farfetched way was a manifestation of those former Roman borders. Aside from religion there are also many other cultural differences between the people above the rivers and below the rivers.
I am way less versed in German history but in Germany you can see the same thing, the Roman influenced areas were Catholic and areas of Germania became Protestant. In Switzerland the areas that remained under Roman control for the longest still speak French, Italian or Romansh, which are languages descended from Latin.
I personally think it is really cool that the regional cultural differences in the Netherlands and Germany have some correlation with what areas were Roman and which weren't. Can you think of other examples of how borders from waaay back still shape our countries and cultures today? Maybe some from your own country? Cheers!
Jesus I was expecting 4 paragraphs tops, I even had to remove a few because I went on a tangent about Germanic warfare and religion in the Netherlands. But yeah here is a too long didn't read in case you need it.
Tldr; Roman border in Netherlands, South Roman North Barbarian. North Protestant South Catholic. Same for Germany. Your country have too?
Source: reddit post