ISIS
The fight for Mosul
The city, which was surrendered to ISIS in 2014, is the second-largest in Iraq. The US has been reinforcing and training Iraqi and Kurdish forces.
One of the terrorist group's goals is to create a caliphate, a literal "Islamic state," and losing Mosul would deal a huge blow to that effort.
Reminder: The outcome of this fight is definitely something the next US president will inherit.