Obama's chemical weapons deal was supposed to get all sarin gas out of Syria, and Obama bragged repeatedly that it did calling it a "triumph of diplomacy" and he was smarter than the "foreign policy establishment" (although clorine gas attacks by Assad continued under Obama, which the authors fail to mention).  If Obama's deal to abandon his chemical weapons redline was legit, Trump would never have been faced with this situation in the first place.

Obama's ultra predictable passivity was what turned Russia, China, and Iran into a super aggressive stance and they took over vast territory with virtually no push back (Crimea, East Ukraine, Syria, Yemen, South China Sea etc.).  

Trump's unpredictability will make Russia, China, and Iran much more cautious and timid becuase what they truly fear is escalation with the US.  Obama never understood this and neither do these authors.