In Capitan America: Civil War, the avengers are in Africa pursuing a dangerous delinquent when a catastrophe in which innocent people die, something that puts all the countries against him.
During the last acts of The Avengers people have died, so that a good part of the countries of the world has gathered in the United Nations and has decided that it is time to end that.
They decide that from now on, the group of the Avengers will depend on the UN, so it will be this organization that gives them the permission to act and will tell them where and how they have to act.
There are Avengers who agree, like Iron Man, but others do not. The rebel group is headed by Captain America, so a war begins between the two groups, a war in which will also be Involved Winter Soldier, a villain who is a friend of Captain America.