Per NATO rules, Ukraine was not eligible to join due to the 2014 invasion by Russia as that was considered a border dispute. Prior to that they weren’t really trying due to Russian influence/ corruption of Yanakovich etc.

As far as avoiding the conflict, Obama absolutely should have taken a harder stance on Crimea and Donbas. Before that Bush should have taken a harder stance against the Russian invasion of Georgia. They were essentially taking an appeasement approach and trying not to get pulled into war with Russia.