What do the people of South Korea, Afghanistan, and Crimea think?

As far as Crimea, it's difficult to say precisely.

They had a referendum in 2014, but that was then, and it was hardly an open and transparent one (armed Russian soldiers occupying the entire peninsula, no international observers admitted).

Ideally, we'd need to hold a new fair referendum to find out. The problem is that even talking about holding one is in violation of the Russian laws on separatism, punishable by up to 5 years in jail. Conveniently, the corresponding bill was signed into law at the end of 2013, shortly before Crimea was annexed. And yes, people have actually been jailed over this.

(Having said that, if there actually was such a fair referendum, I would bet on them staying in Russia. But the percentages would probably be lower than last time.)

There're millions of people living in all these places and it's a typical mistake to think that they have the same uniform opinion. You can take any subset from the population and create an illusion that they all support or reject something, like US forces presence. I see this in news every time I open them or watch TV.