How do county, state or international boundaries influence culture? For example, the Boston Red Sox are based in Boston, Massachusetts, but they also have fans in Rhode Island, Vermont, New Hampshire and Maine. Things like that. This question was raised when I was looking in a book at a U.S. accent map and noticed that the area that denoted the Southern accent extendened just a little bit into Mexico.
Also, French is spoken in parts of Vermont and Maine on the border with Quebec.
Terrain can influence territory. Around NYC, Westchester and Long Island have noticeably different cultural vibes - accent, Yanks vs. Mets fanship, probably a lot of other stuff - partly because they’re separated by Long Island Sound, partly because the basic population stock is different. Bronxites tended to take “flight” to Westchester, Brooklynites to Long Island.
I’ve noticed a world of difference between Dallas County and Tarrant County.
Dallas is thought to be the last Eastern city and Fort Worth the first Western city. Dallas has a very fast pace to it, urban and modern. Ft. Worth seems to be a cowtown and proud of it.
Here in Tennessee, it’s mountains, hills and plains.Most of the eastern part of the state has the culture of the Appalachians…fewer chances for the working class for education, big gap between the working class and professional class, etc. The culture tends to be polite but reserved with outsiders (historically, with good reason).The hills is more of the old south culture with native professionals, working class and poor mixed with a lot of of transplants and a growing immigrant population. The education level is usually higher and the culture more mixed. The economic base tends to throw folks with differing backgrounds together on many levels, so we’re a little more accepting of lifestyles other than our own. The west is pretty agricultural outside of Memphis…small towns, light manufacturing and farms. A lot of attitudes are typical of the southern midwest.
So yes, terrain does have something to do with it in a lot of places (here, at least, but I can think of others).
My state of New South Wales is about 313 000 square miles, as compared to Texas’ 262 000. This becomes important when the state capital of Sydney is about as far from any interstate borders as you can get - if you think of the state as a misshapen square with the northern, western, and southern edges adjoining other states, and the eastern edge being the coast, then Sydney is halfway up the coast. On the other hand, Brisbane, the state capital of Queensland, is very close to our northern border, Melbourne is closer to the southern parts of the state than Sydney is, and Adelaide is the closest city to the far west. All these cities are in other states, and their influence is obvious, and extends quite deeply into the interior of the state. Several hundred miles before the border, you’ll see people preferring the interstate beer, picking up interstate TV, following interstate sporting teams, sending their kids to interstate universities, and so on.
This adds to the resentment of Sydney, and there are frequent calls for borders to be shifted (but we in Sydney just tell them to shut up and keep paying their taxes. 
). The far western city of Broken Hill even goes as far as to defy the New South Wales government, and opts to operate on the South Australian time zone.
I live in what used to be the Ottoman Empire, so of course there was no border between Bulgaria and Turkey for 500 years. An intentional colonization program brought thousands of ethnic Turks here. When Bulgaria became independent in 1878, hundreds of thousands of Turks ended up on the Bulgarian side of the border. Today they make up about 10% of the population, and in some areas it’s much higher. (About 35% of the population in my town is ethnically Turkish.) Turkish culture has had a HUGE impact on Bulgarian culture - both pop and folk music have a very obvious “Middle Eastern” sound (there’s also something called “pop folk”, which is kinda trashy but great to dance to), there are lots of Turkish borrow words in Bulgarian, etc.
During the mid-1980s, the communist government announced that Bulgaria’s Turks weren’t really Turks at all, but “Turkishized Bulgarians” and set off on a campaign to Slavifiy them. They were given new, Slavic names, mosques were closed, and Turkish language classes shut down. Hundreds of thousands of Bulgarian Turks crossed the border and settled in Turkey. Interestingly (well, to me, anyway, but I’m a nerd), many of these eventually returned to Bulgaria, because they found that Turkish Turks looked down upon them as somehow culturally inferior. (The Turks in Bulgaria are about as religious as everyone else in Bulgaria: not very.)
Right now, my little town is filled with Turkish tourists, and I was really puzzled that a lot of them seem to be able to speak Bulgarian, too. I only recently connected the pieces of the puzzle - they’re people who left Bulgaria for Turkey, back visiting family still here.
Also, in my part of Maryland, most of our media comes from DC.
I’m hoping you mean east Texas and West Texas, not the whole country.
No, if you are here you pretty much think of that as the division of the whole country–East Texas is part of the SE if it’s part of any region of the country, and West Texas is Western. And Dallas-Ft. Worth is as good a place to divide it as any.
Or Minneapolis and St. Paul.
Cross the Irish border, just a few minutes drive from my parents’ house and the most obvious sign of change (except of course for the road signs 
) would be the accent. Strange how it changes so much across the border, it sounds so, well, Irish. Northern Irish really is a different kettle of fish altogether.
I got on a train yesterday in Cologne, Germany and got off less than two hours later in Liege, Belgium. The buildings were different, the food was different, and everyone spoke French. While it wasn’t a touristy town, it was still pretty close to the German border. Still, my friend and I never encountered anyone who spoke German, or English.
I suppose if you have to draw a geographical east/west boundary line, it might as well be there. But not culturally – even Chicago doesn’t have the feel of an “eastern” city.