What are reasons the united nations exist?

The forerunner of the United Nations was the League of Nations, an organization conceived in similar circumstances during the first World War, and established in 1919 under the Treaty of Versailles "to promote international cooperation and to achieve peace and security."

The United Nations (UN) is an intergovernmental organization to promote international co-operation. A replacement for the ineffective League of Nations, the organization was established on 24 October 1945 after World War II in order to prevent another such conflict.