Let's be honest here the US was holding the valuable real estate in Syria, the rest of it was junk, if they had wanted it they'd kept it.
Russia was involved in Syria then they withdrew when it seemed Assad was getting a upper hand on the rebellion. Then things turned concerning ISIS and the infiltration of them into Syria, whereas Russia, the US and Syria all worked together to contain the threat. You will find plenty of headlines to that effect. This was never about Russia going to go up against US forces in defense of Syria for Assad...that never happened. Basically they are like a bunch of babysitters between the warring factions...the object was to remove ISIS (regardless of the debate of how they sprung up), that objective according to Trump has been achieved...now the question is whose the winner? Assad won against the opposition, every war someone has to be the loser, according to Trump we don't need to be there, we don't get to pick the winner.