Even beyond militaristic action, there are so many low hanging fruit that weren't picked. It took until 2019 to sanction any appreciable number of people in the Syrian regime. Many relatives and employees of the Syrian regime are happily flying back and forth between Damascus and Europe. Their illicit assets are safely parked in foreign bank accounts. They're sending their kids to LA and San Francisco, buying $2M homes with straight cash and going to the best private schools. Severe sanctions should have hit anyone even remotely involved with the Syrian government as soon as it became clear how many innocent people were going to be killed. Foreign assets should have been seized as damages to be distributed to the Syrian people in the future.

Sure, but I find these sorts of sanctions to be ineffective at the stated goals of preventing killings, etc. And Russia and China, at least, were not planning to participate, leaving a wide open avenue. The UNSC couldn't pass sanctions because of their vetoes.

America and Europe can only be hamstrung by Russia's veto if they allow that to happen. There are many global measures that could have helped the Syrian people outside of any action by the UN Security Council. Nonetheless, I agree with you that America left a void of leadership, and that allowed Russia to walk in unimpeded. The Syrian people were about to oust their dictator until Russia came in with reinforcements. My point is, America wouldn't have even needed to send in the Air Force. The most important course of action would have been to prevent Russian entry. That could entail a guarantee that "if Russia or Iran enters Syria, we will create a civilian safe zone using the Air Force." The safe zone would have both saved civilian life and protected the FSA from aerial bombardment.

Arming FSA was a good option for much longer than just the first year (and still is). Obama’s advisors (including Clinton at State and Panetta at DoD) advocated this, but were overruled. A large factor was his overwhelming desire to secure a deal with Iran, which even caused him to walk back from his “red line” when Assad used chemical weapons in 2013.

The pattern you describe is very similar to what happened in Europe around WWII. Places like Poland and Slovakia produced local uprisings (against the Nazis). The Allies basically sat idly by while those got crushed, enabling the Soviets to then move in and create the Eastern Bloc.