Because, unfortunately enough, the Ukraine, Armenia, Georgia and all others are small countries whose population doesn't exceed 10 million and have militaries and economies incomparable to that of Russia.
Actually Ukraine has a population of 44 million people. Also, its GDP is ten times greater than the GDP of North Korea.
South Korea is a democratic republic of 50 million whose economic size and competence ranks one of the highest in the world, and wields a large(small only relatively to China and Japan) to Russia.
True, but Russia wouldn't be invading South Korea; they'd be invading a rapidly-collapsing country that borders South Korea... there's a bit of a difference here.
Furthermore all other regions have reasons why Russia'd want them - historic naval bases, oil reserves, etc. What does North Korea offer? Free labour? They could buy that shit much easier. Warm-water harbour? Vladivostok suits them extremely well for that situation.
No, the main reason Russia took over places like South Ossetia was to prevent the areas on the borders of their country from falling into NATO's sphere of influence. (South Ossetia is a tiny landlocked area with no naval bases or oil reserves afaik)
'tis exactly the same here: Russia would want to stop a close U.S. ally from gaining territory near them.
The question on refugees concerns the Chinese as well. Why does the Chinese wish to meddle with a potential crisis, whether it be a riot, anarchy, food shortage or epidemic, for the sake of gaining a few more miles into the peninsula? So their map looks nicer?
Same reason as Russia: to reduce the spread of US influence.
Occupying parts of North Korea would also give China and Russia a huge bargaining chip: they could offer to cede their occupation zones to South Korea if and only if South Korea removes the American military presence from their country.
I wouldn't be surprised if a lot of North Koreans would rather live under Chinese rule than live under South Korean rule (anti-American propaganda at work), so establishing an occupation zone should be easy.
The US does not leave its friends bound in blood alone. And South Korea has shared that fate for eight decades now. And South Korea has declared the entirety of the Korean peninsula its sovereign territory since 1948.
Does the USA even recognize South Korean sovereignty over the entire peninsula, anyway? If not, the argument that the US would "defend rightful South Korean land" (i.e. the North) becomes a lot weaker.
And yes, I'm definitely suggesting the US can engage in ground warfare over North Korea if the situation arises. Because even without them South Korea's going to do so anyway.
Yes, the US will fight North Korea on the ground. I'm not convinced that they would fight China and Russia on the ground. I think it would be more of a Crimea-like situation: trade sanctions and condemning, but no actual war.
