Unilaterally withdrawing from Syria may not be the best short term move but it may be the best long term move, not for the fight against ISIS, Al Nusra, Iran / Russia action in the ME, etc... but in the fight against the Swamp, Deep State, Globablists, etc... that the worm has turned and a sizable portion of America thru their elected representation be it in the Legislative or Executive branch is not going resource with blood and treasure endless conflicts that have no readily achievable goals or relevant goals for America, no matter if there are laudable side effects to a Pax America enforced order.
Like it or not we have hit the end of our ability to protect X number of others, to deter for X number of others, to fight for X number of others, not really for a lack of material resources but spiritual resources to be willing to do so. Explain in a clear way to the forgotten people of the Rust Belt, Bible Belt, Urban Jungles... that denying victory / deterring Russia, Iran and their proxy Alawite pawn in Syria which has no natural resources we purchase, no strategic role in trade, defense or significant cultural position to America is a reason we should spend 15+ billion a year bombing random jihadis half-way around the world when we have 8,000+ fighting age males massing on our border and demanding entrance or else... explain how this time, this operation will resolve something, will lead to a situation that is marginally better for them or for us, explain how just another 15 billion over there versus spent over here on anything is better...
Indeterminate involvement may have some positive effects for others, some are probably worth defending but America's resources are finite. Even though the material cost for this particular operation is / was not onerous per year in the grand scheme of things, the fact that it would be never-ending has caused the American Nationalist movement thru its somewhat brash and inarticulate standard bearer to say no more. No matter if you believe we should be militarily engaged in many places around the world, the fact that 67+ million Americans said no more, should give you pause.