Speaking of the current Russia-Ukraine crisis, here is an interesting but less visible international legal dimension to the story.
Ukraine used to be part of the Soviet Union, during which time it had possessed huge stockpile of nuclear weapons arsenal – actually the third largest stockpile in the world at the time. Russia would not have ventured into Crimea today had Ukraine maintained possession of those nuclear weapons. What happened in 1994 was dramatic, and a bit embarrassing for Ukraine.