Obama withdrew US troops because there wasn't a military solution for a sectarian war. Its Muslim vs Muslim, Sunni vs Shiite. Its been going on for many, many years; possibly hundreds of years. Lets not forget The Kurds are in it also for many, many years. Its a religious war and we have no business being there.
What Trump did was give the keys to his buddy Vladimir.
There is also a human rights issue with so many imprisoned by the Assad regime. Conditions are horrendous. All facilities are overcrowded. There are reports of torture, starvation, unsanitary conditions. People dying by the hour of infections resulting from torture. People being flogged, water boarding, electrocution.
#45 also gave a free pass to the Turkish govt. Turkish troops have entered Syria, ready to impose their will on the Kurds, a US ally. Not much of a surprise considering the Trump corporation has a total of 119 investments in Turkey, including the Trump Towers. A group of wealthy Turks have invested over $400M in the development of Trump Towers.
#45 looks to further enrich himself