Liberal Position
ISIS was referred to by Obama as the JV team and not a serious threat to the U.S. or the world. The Obama/Biden policy was to "Contain ISIS" and keep them in Iraq, but not to go after them in urban warfare. From Washington Monthly their Containment stategy and from CNN's explanation.
The theory was to cut off the expansion efforts and let them suffocate themselves. This approach did not work and ISIS grew larger and stronger. ISIS grew and took over land and began running towns and cities in Iraq, butchering women and children, raping women and killing the citizens of the towns they controlled. This collection of land was known as the Caliphate.
ISIS was one of the biggest threats facing the world in 2016.