Syria is complicated. This article is by a Cato Institute author with some strong opinions I am not necessarily endorsing, but it does break down some of the key players.

• Fact: The U.S. government has ground troops inside the borders of Syria, a sovereign country with a seat at the United Nations, and it does not have the permission of that government to be within its borders. The two countries do not have friendly diplomatic relations but nevertheless, neither side claims to be directly at war with the other.
• The stated reason for U.S. troops entering Syria was to fight the Islamic State group. By many accounts, that objective has been achieved. It is also worth noting that by some accounts, the reason that group formed was blowback from the 2003 U.S. (mostly unprovoked) Iraq invasion.
• There are, however, regular “drone and rocket attacks” on U.S. troops by militant groups “aligned with Iran and Syria”.
• The Syrian government is publicly anti-israel, and the U.S. government is obviously an ally of the Israel government. This article doesn’t mention it, but Israel is also known to be carrying out regular strikes against groups on Syrian territory that it considers threatening and/or Iran proxies.
• The government of Russia is allied with the government of Syria. The United States presence in Syria is therefore “discomfiting” to the Russian government according to some. Russia has troops on the ground in Syria with the permission of the Syrian government. The U.S. and Russia are not directly at war in Syria or anywhere else, but there have been confrontations, provocations, and “harassments”.
• The U.S. government supports military forces of the Kurdish ethnic group, which some say serves as a de facto government controlling territory in this area. These Kurdish forces are openly engaged in military hostilities with Turkey inside the borders of Syria, which is a NATO member and declared U.S. ally.
• The government of Syria and the government of Iran are allies, and the U.S. government is openly very hostile to Iran and accuses them of interfering with politics and funding wars and terror groups throughout the Middle East. The governments of Iran and Israel are also openly hostile, of course, with nuclear risks for the region and world.
• Some say the U.S. is trying to “bring Assad down” or “steal Syria’s oil”. I don’t know how real these claims are or whether either represents any sort of official policy (well, certainly not the latter, and deploying the U.S. military to “steal oil” tens of thousands of miles away simply can’t be a viable business proposition. This one does not pass the logic test.)

There – I don’t know that I “explained” it, but I don’t know that there is anything to explain. We are there because they are fighting us, and they are fighting us because we are there. There are at least four distinct conflicts happening in the same geography – U.S. vs. Russia/Syria/Iran/islamist groups, Israel vs. Syria/Iran, Syria vs. Kurds, Turkey vs. Kurds. What a mess. Even Donald Trump wanted to get out of Syria, probably for what I would consider the wrong reasons. Let’s get the U.S. military out and the diplomats in. Where is Jimmy Carter when you need him? Who is the next Jimmy Carter – Obama maybe?