Absent additional information, and taking into account the original categorization of the question under "History," it will be assumed for the purposes of providing an answer that the "UNO" in question refers to the United Nations Organization.
Following the end of the First World War (1914-1918), U.S. President Woodrow Wilson advocated the establishment of an international organization that would provide a forum for nations to argue and negotiate over their differences without resorting to another war. The result was the League of Nations, which formally existed from 1919 to its dissolution in 1945, by which time its ineffectiveness had become apparent. The United States, which conceptualized and agitated for the League's creation never became a member because of congressional opposition to formal participation in international organizations that might constitute entangling alliances or impede U.S. freedom of action.
The League's failure did not inhibit President Franklin Roosevelt from similary advocating the establishment of an international forum for conflict prevention and resolution. Early in the Second World War, the United States, with 25 other nations, united to defeat the Axis powers (Germany, Japan, Italy) under a Declaration by United Nations. This declaration constituted the introduction of what would become the United Nations Organization. The organization's charter and structure began to take shape during the wartime conferences of the leaders of the Allied nations. On April 25, 1945, the United Nations Conference on International Organization was held in San Francisco, California.