This widely recognized Romanian military analyst and expert explains why the US military operations in Syria were essentially checkmated by Russian-Syrian military coordinated with Turkey's obsession to eliminate all Kurdish forces. One must understand that the Syrian Kurds permitted their forces to be used by the USA which then established numerous military bases on Syrian territory that the Kurds controlled. By doing this, they allied themselves with the US Empire that was trying to destroy the Syrian government.
The US military air bases in Syria consist essentially of troops for special operations. By this we mean a light infantry, without any armour or support. They could not therefore ward off any land attack carried out by the Syrian army supported by the Air Force. Having understood that the US Air Force will not be able to pass the Syrian anti-air barrage without unacceptable losses, any US intervention becomes inappropriate. This is why the US has just announced that it will start to withdraw 2,000 soldiers from Syria . At the same time, Turkey, supported by Russia, is getting ready to launch a new offensive against the YPG in Northern Syria. These new circumstances ensure the Syrian Army will fight on the side of Turkey. The YPG, trained and supported by the United States, is quickly losing all the territories that it had taken from the Islamic State which itself had taken from Syria.It wasn't Trump abandoning the Syrian Kurds as corporate media pundits and government official declared, but US military analysts who, no doubt, saw that their continued military occupation of Syria was now untenable.
(You may also be interested in Elijah J. Magnier's take on the withdrawal of US forces by reading "USA announces withdrawal from the Levant: time for all parties to rethink their next move".)