(Though I think it’s not irrational to worry about instability you don’t know, even if the previous situation was also one of instability)
-facilitated jihadists to Iraq
-forcibly displaced ~50% of Syrians to Lebanon, Turkey, Jordan, Iraq, Europe
-fuelled the rise of ISIS
-enabled Iran to extend its axis
-enabled Russia to extend its malign influence
(Though I think it’s not irrational to worry about instability you don’t know, even if the previous situation was also one of instability)