Saw online how someone named Belarus as Belorussia. That’s what triggered me to write the following text.
Belorussia is a non-state. That’s how Russians call Belarus.
Belarus is the official and correct name, the origins are from Rus. The medieval country, that was on the territories of modern day Poland, Ukraine, Belarus. Russia stole that name (Rus’ or Ruthenia), just recently — in the historical terms — a couple of hundred years ago. Their country was named Muscovy. About that time they invented their artificial language, which is mostly stolen from Belarus and Ukraine. During the occupation (1918—1991) period, they did their best to eliminate their (Belarus, Ukraine, but also all the other nations they enslaved) culture, language, most prominent people. The word to search for is genocide, if you feel like willing to explore this topic more. Belarus is still de-facto occupied by Russia, never recovering from Russia‘s barbarity of XX century, that’s why it’s a pathetic pro-Russian country now.
Ukraine too was under huge Russian influence since 1991, and technically only since 2014 (the revolution of dignity, Russia seized Crimea) they do fight for their independence. In fact, current war is the independence war for Ukraine. If they won, they’re just another thriving European (EU) democratic state. If they to lose, Estonia, Latvia, Lithuania, Poland are the next in line. And Ukrainian people would be forced to fight for Russia.
Modern Russia in its current form cannot exist without Ukraine. Without Ukraine, there’s no Russian empire, there’s no USSR. That’s why Russia tries so badly to occupy Ukraine and name it Russia. Without them, Russia is just two cities, Moscow and St Petersburg. That’s why they desperately call them brotherly nations, Ukraine and Belarus. (And some other neighbouring states too.)
During the XX century, Russia invented derogatory names for all the countries they enslaved. That makes Belorussia or Byelorussia (notice Russia, not Rus, which is completely different country, despite Russia trying to claim the name). Also, the names are ‘on the Ukraine’ (Ukraine has no ‘the’ article, and is ‘in Ukraine’), that way they try to mock the name into the name Periphery (this word sounds very similar to Ukraine in Russian language), that way they present Ukraine as some distant and rural periphery, while Russia itself is a periphery. (Basically everything Russia claims of others, is actually about them and about what they do to others. See [Accusation in a mirror](https://en.wikipedia.org/wiki/Accusation_in_a_mirror] for that)
Also, derogatory term for Baltic states is Pribaltics (in the same fashion to Ukraine, making it rather a territory), and also I know of Moldova, they call it Moldavia. And Turkmenistan, they call it Turkmenia (in the same fashion). Maybe there are other countries, but I’m unaware of them, since I haven’t been there. Russians claim that innocently, that that’s just their language, that’s how they always named these countries. So yeah, technically, we cannot tell other nations how they should name our countries. Except when we actually can. Recently (after 2020) some countries (I’m aware of Germany) renamed Belarus from being literally White Russia, to Belarus. Which historically can be translated as White Rusyns, but not White Russians, because Russia never existed by that point in history. Also see Turkey becoming Türkiye. It’s very similar thing, as far as I’m aware, and I believe calling them after the bird is disrespectful. Especially when they explicitly asked how they want their country to be called.
Hope that’ll help someone who is unaware why some people call these countries wrong names. I’m not giving links, because everything I wrote is very easily searchable and makes sense only for those who are far from Europe and the context to understand and/or care.