In brief, why not:
- It is a struggle to understand a way in which an attack on Syria is legal, barring an act of Congress.
- It seems unlikely that airstrikes alone will be sufficient to unseat the Assad government.
- The Assad government has sufficiently strengthened its position in the last year such that it seems unlikely that airstrikes will tip the balance in favor of Syrian rebels.
- There is no reason to believe that President Trump has sufficient self-control to manage a limited air campaign that fails to destroy the Syrian government.
- There is no reason to believe that a strong constituency exists in Syria for a prolonged American ground occupation.
- The Syrian rebels are deeply factionalized, and have become increasingly radicalized; it is not obvious that the Assad government would be replaced by a central government at all, or that such a government would be meaningfully preferable to Assad.
- While Russia is unlikely to directly oppose US strikes, the risks of accidental escalation are nevertheless present.
These are all issues that the Obama administration wrestled with for five years, to no particularly good resolution. They are issues that Hillary Clinton had no particularly good answer for. They have not changed for the better since Trump’s inauguration.