Obama didn't "let" Putin do anything. Putin annexed Crimea, and Obama responded, but he did it without using military force. There was a response, and Obama did say that what Putin did was wrong. Obama didn't let Putin take Crimea as you claim. Matter of fact, Obama offered aid to Crimea, but not military solutions.
US president insists military solution not an option, saying pressure and diplomacy are the way forward in Crimea dispute
www.theguardian.com
Guess you're too busy believing the bullshit laid out by Trump and echoed in the right wing talking chamber, you do seem to like opinions that reflect what you believe, even when it's not true.
However, it is your buddy Trump who has been knowtowing to Putin when it comes to Crimea and the Ukraine.
President Donald Trump mocked former President Barack Obama as being weak on Russia while Moscow illegally annexed Crimea from Ukraine in 2014.
www.businessinsider.com
See what I did there? I provided links of actual proof of what happened to back up my point, not just someone else's opinion.