The “war” in Syria has roots in our invasion of Iraq. We created millions of refugees who went to Syria. As happens all over the world where lots of refugees settle in a foreign country, it did not go well for Syria. Their attempts to crack down on problems led to the civil unrest.
Saudi Arabia saw this as an opportunity to overthrow Assad, and supported forces with money and aid to start a civil war in Syria. The US at the behest of the Saudis (and eventually Israel) created ISIS and supported the efforts to topple Assad.
Of course it didn’t work out the way we planned. Russia came to Syria in support of a long term ally and Assad remains in power.
President Trump directed the CIA to stop supporting the Syrian rebels.
It appears that our CIA and military are willingly ignoring the Commander in Chief and still supporting the rebels. Read more here.