Both Russia and US signed the 1994 Budapest agreement guaranteeing Ukraine's boarders as the existed at that time. This included Crimea. History before that is irrelevant.
That Putin fails to honour his international obligations is not surprising; he is pure evil.
One would have hoped that the US had more integrity, but Trump is dragging US down into the gutter along with Putin.
All Americans should hang their heads in shame at what their country is becoming. US's word now means nothing.