alt Hacker News> [0]: My favorite "fun" fact is that the Holocaust was legal. You can kill a couple million people if you write a law that says killing those people is legal.
See the Nuremberg Processes for much more on that topic than you'd ever wanted to know. 'Legal' is a complicated concept.
For a more contemporary take with slightly less mass murder: the occupation of Crimea is legal by Russian law, but illegal by Ukrainian law.
Or how both Chinas claim the whole of China. (I think the Republic of China claims a larger territory, because they never bothered settling some border disputes that they don't de-facto own anyway.) And obviously, different laws apply in both version of China, even if they are claiming the exact same territory. Some act can be both legal and illegal.
Yep, legality is just a concept of "the people who control the people with the guns on this particular piece of land decided that way".
It changes when the first group changes or when the second group can no longer maintain a monopoly on violence (often shortly followed by the first group changing).