The law might be that the democratically elected leader of the Ukraine went to Putin and said "help me".
The US is helping those who aren't democratically elected.
The US goes around the world bleating about democracy, and yet help the coup d'etat in Venezuela in 2002 to oust democratically elected leader Chavez, yet supports the Saudi rulers who wouldn't know what rights, democracy or freedom were if they hit them in the face.
Go figure.
Bullshit! Exactly what law does Putin have to justify his attack on the Ukraine? Is it the Buddapest Memorandum?
What law? Why does he need a law?
If Obama goes and asks the Canadian PM to send in troops to help out, what law does Obama need?
The Ukrainian president would clearly have the power to protect the Ukraine from invasion, insurrection, coups and other such disasters, right?
So that's the power he needs, the power he was elected to carry out, and what he appears to be trying to do.
The problem is, the US is willing to ignore the democratic issue here.