The simplest description is :Obama showed weakness on Syria so Putin exploited it in Ukraine.
Lesson, don't make threats, unless you mean to back them up. Obama, or his advisers should have known better.
To be fair, the threat of an American attack under President Obama’s administration persuaded Syria to destroy a significant amount of chemical weapons and the materials and equipment used to make them.
Short of that ^ major capitulation happening, there would have been a significant regime changing U.S. attack on Syria - not like the pathetic attack that the corrupt orange traitor launched against Syria.
I would also submit that the Russian annexation of Crimea wasn’t influenced by events involving Syria at all. The potential sudden loss of the Russian Fleet’s base at Sevastopol after the Ukrainian revolution of dignity was the primary / singular impetus, imho. And a U.S. led attack to forcibly expel the Russians from that base (and the peninsula) at that time would likely have resulted in a nuclear exchange.
I go back to my contention that President Obama handled the Crimea incident in a clear eyed pragmatic way back then - similar to how President Biden is handling things now.