There are hundreds, if not thousands, of regions and countries which have been transferred to a foreign power through "international agreement". Probably even your own birthplace at one time or another in history. And those agreements almost never has anything to do with what is right or justified, or taking into account the self-determination of the population.

So I don't understand how anybody can support "international agreement" by default. I certainly don't. Regarding Crimea I don't have an opinion, because I don't live there.

Germany was split into two by "international agreement", just as an example.