Obama can be blamed for many things.
Putin’s aggression is not one of them. He was going to illegally annex Crimea for its naval yards and connect a land bridge from Russia through E. Ukraine to those naval yards, regardless.
This while one of the powers behind Obama, George Soros, poured money into the Ukraine to stir chaos and impossible expectations.
One of those impossible expectations was that Russia would walk away from Crimea - when Russia took it from the slave trading Ottoman Empire 250 years ago and has maintained its presence there since. This Russia did even after losing the Crimean War and even (through military treaty) after the collapse of the USSR and Ukrainian independence.
Yes, the megalomaniac bumbler occupying the White House IS to blame for the Ukraine also.