It isn't a legal justification, but it helps people to understand why this is happening.
Most of the people in the Autonomous Republic of Crimea have no problem with the Russian military because they are Russian themselves.
I predict that Russia will have troops there for a long time.
Who is going to make them leave?
Not the Ukraine, not NATO, and not the USA.
It's a waste of time talking about this. It is what it is and it's not going to change.