I admire Mattis but being in Syria was accomplishing nothing but prolonging the civil war and keeping floods of refugees going into Europe for as long as it stayed unstable (Which we were doing our level best to make permanent).
The US never had enough forces committed to do more than keep Assad from winning. Committing real main force ground units in big numbers was never an option -
We don't have them to spare;
Their logistics would depend on transport through Iraq, where thanks to Obozo's precipitate exit without fighting for a SOFA, the Russians are gradually edging us out of any influence;
The electorate will turn on any commitment that takes more than a Presidential term (Two at tops) to wind up, and his party will lose to a 'Get the US out' opposition candidate.
Look at the Arab Spring that Obama and Clinton wanted so uch credit for. Yeah, a lot of relatively secular autocrats were deposed, but they were all replaced by religious fanatics or failed states with local religious fanatic warlords, except Egypt...which was briefly taken over by a Muslim Brotherhood government, before an army coup...so net-net, the Egyptians didn't end up any better off, they just didn't end up any worse.
The intelligence and defense establishment (Including Mattis) is still married to regime change in Syria, which was even less-effectively pursued by the previous Administration, but it was not the announced policy basis for our present commitment in Syria.