Perhaps Trump has some sort of deal with MBS and thus Saudi Arabia on this. Syria/Iran/Russia control of the pipeline could be important in the balance of power. The presence of troops in the Middle East may be a deterrent or a pre-position depending on what happens.
Trump does not change position unless there is a reason for it.
there was a coalition that came to the aid of the Syrian Freedom fighters that included lots of Syrian military. Barack Obama for reasons never stated abandoned the group in the middle of the war. The date in my mind is 2014.
Why? the media never explored the events and never got an answer.
Isis was the result of the Obama abandonment.