The UN or United Nations are a group of nations that come together for one purpose. That is to maintain order and peace among the countries of the world. It was established after World War II and they are known for trying to keep the peace or make peace. With almost 200 countries participating, the United Nations is well represented. Its headquarters is located in the United States.
However, there are several other offices around the world. If the United Nations started fighting ISIS, then it would defeat the purpose that they stand for. Instead, they are trying to stop the terrorist groups, but they are trying to do this peacefully. It is up to the individual countries to use force, but not the United Nations.