Yes, the war is indeed very complicated. One week ago the Turkish president said this: ''Within a week we will enter Afrin and Manbij if the terrorists won't withdraw''.
Alright.. One week is over and still no ground operation, the fun thing is that the Turkish president has been threatening for a year now and that there are still people over there that believe in him and the operation.. 
Red: Assad/Russia
Green: Rebels/Nusra
Yellow: Kurds (YPG/PKK)
Black: ISIS
It all started when Turkey claimed that the YPG was the same as the PKK, so the Syrian kurds (yellow on the map) are terrorists in the eye of Turkey.
Back in 2014 and 2015 the Turkish border was completely in the hands of ISIS. Turkey did not do anything against ISIS, and then out of a sudden the Kurds came. What happened? The Kurds took the whole border with Turkey from ISIS. And now Turkey claims that the kurds are terrorists and want to attack them. Very weird yes.
Why did Turkey not attack ISIS back then? If they did they had the control over their whole border and there won't be any Kurds either there.
Turkey is really playing with fire and the attack on Afrin could lead to a whole border war with the YPG. And we all know USA supports YPG and also has soldiers over there.