One often hears defenders of the US presence in Syria claiming that the US is there to fight ISIS. In contrast some opponents claim that the US created ISIS.
I won't enter the question whether the US was involved with the creation of ISIS here. Those claims are based on quotes from US official reports and some statements by US politicians. Instead I will discuss here the timeline of US involvement in Syria and what it actually did (and didn't do).
- The US got involved after ISIS had made great advances in Syria and threatened the Kurds in Kobani. If the actions of the US had been purely logical it would have gotten involved earlier. It would have given some arms to the Kurds and dropped some bombs on ISIS and that would have been enough to turn the chances on the battle field. Instead it waited until there was an international uproar that gave it an excuse to enter Syria and a Congress resolution within the US. Without such an excuse its entrance would have been hardcore illegal.
- After the US got involved they and their Kurdish allies conquered much of Northeastern Syria. After that they stopped. The Turks occupied Jarabulus and blocked the road to the West and further south there lived hardly any Kurds so the SDF wasn't enthusiastic about spilling blood to conquer those areas.
- Then ISIS conquered Palmyra in May 2015. To come there it had to cross 200 km of desert. Yet the US didn't drop a single bomb to stop them. Shortly afterwards pro-Russian media reported that American diplomats were preparing Western capitals for the fall of Damascus to ISIS.
- After Russia got involved one of its first actions was to go after the oil trade of ISIS that provided much of its income. A few bombs did a lot to stop that trade. The US had never bothered to do so.
- In September 2016 the US - allegedly by mistake but many people don't believe that - bombed Syrian government troops in Deir Ezzor. This almost led to the fall of this government held enclave to ISIS.
- Some time after Russia had become involved and the chances on the battlefield had changed the Syrian government started an offensive from Aleppo eastwards towards Raqqa. Suddenly the US felt the need to go south towards Raqqa too - at the end of 2016. Obviously preventing that Raqqa came in the hands of the government was more important as getting ISIS out of Raqqa.
- The first thing the US/SDF coalition did near Raqqa was to cross the Euphrates and occupy Tabqa and its dam and surroundings. The obvious goal was block all the roads so that Syrian Army offensive couldn't go further south (further from the river there was desert). In the end the Syrian Army outsmarted them. But there were some very tense moments.
- The attack on Raqqa lasted many months and completely destroyed the city. No other city in Syria is so destroyed. According to the Kurds the US refused to give them advanced weapons that would have made them more effective against ISIS.
- The Syrian Army moved further south and conquered Deir Ezzor. Shortly south of the city it crossed the Euphrates in an effort to conquer the oil and gas fields there. However, they encountered fierce resistance from ISIS and didn't come far. The US - that still was stuck near Raqqa some 120 north - suddenly felt the need to move to the area too. In contrast to the Syrian Army they hardly faced any resistance from ISIS and could move tens of kilometers a day. It was the SDF/US coalition that ended up controlling the oil and gas fields - thanks to ISIS.
- The Syrian Army moved still further south and ended up controlling the whole west bank of the Euphrates. However, the US forbade it to attack ISIS on the other bank of the river and threatened to bomb them if they tried. This alone delayed the defeat of ISIS in Syria by many months.
I don't know what the official mission of the US Army in Syria was but it doesn't look like defeating ISIS was the top priority.