system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why isn't the US happy about Russia being in Syria? Isn't it good for us if they crush ISIS at their expense and with their soldiers?
gpt
There is a pretty complicated war going on. It's more complex than this, but there are essentially 3 sides fighting at once:

FSA - Rebels against Syrian government, USA-backed

Assad Syria - Syrian government, Russia-backed

ISIS - Radical muslim, both USA and Russia enemies

When the USA fights ISIS, they of course focus on fighting them in ways that benefit the FSA... attacking ISIS forces threatening FSA, etc. Russia, however, fights ISIS in ways to hurt the FSA and support Assad Syria.

So, while both USA and Russia are anti-ISIS, they are also indirectly fighting against each other by supporting their side in the war.