Conversation so far:
when was the united nations founded and why
The UN Conference on International Organization opened in San Francisco, 25 April 1945 and is responsible for maintaining international peace and security.
Why was the United Nations founded?
The United Nations was founded to replace the League of Nations following World War II and to prevent another conflict.
When did the League of Nations dissolve?
The final meeting of the League of Nations took place on 18 April 1946 in Geneva and finally dissolved itself on 31 July 1947.
Why did the League of Nations fail?
The onset of the Second World War showed that the League had failed its primary purpose, which was to prevent any future world war.

Question: How are the two entities different?
Self-contained rewrite: How are the League of Nations and the United Nations different?