Many are tripping up over Syria because they think in formulas about US regime-change combined with what can only be called an orientalist attitude toward Arab regimes. It’s simple-minded politics including from those who call for “nuanced” analyses of Assad’s dictatorship.
The differences between Iraq under Saddam Hussain & Syria under Assad are substantial & so is US intervention. The decisive difference is that Syria has a popular revolution against Assad as part of the Arab Spring; Iraq did not have a mass uprising against Hussain. The resistance in Syria to Assad, Russian, Iranian, US intervention is primarily coming from the revolution. The militias are both pro-Assad & pro-revolution; others are psycho operations.
If you try to impose a US regime-change matrix on both Iraq & Syria & derive your politics from that, you will end up where all Assadists & orientalists do: denying political agency to the Arab regimes at the same time as becoming the champions of dictatorship.
Does the US have a strategic orientation to the Middle East? Of course. But the Pentagon doesn’t call all the shots. Arab regimes have their own interests at odds with the US & these need to be brokered with a long-term approach. For now, the one thing they agree on, the collaboration between Assad, Putin, & Trump in Syria rests on, the bombing of Yemen, the massive repression in Bahrain & Egypt, is the need to destroy every vestige of the Arab uprisings against dictatorship. They can all abide tyranny but none abide democracy.