The question of "what if" is an incredibly loaded question.  So instead we are going to talk about the accomplishments that were brought about by the creation of the United Nations.

What if the United Nations wasn't here:

In June of 1948 the first UN observer mission is established in Palestine as a peace keeping force.  Then in January 1949 an UN envoy secures a ceasefire between the new State of Israel and Arab States.

During the Korean War the UN was an acting force in banding together many countries' aid against the North Korean invasion. 

The UN helped to set up the government in the newly independent Congo.  There was much disharmony when the French government pulled out and the UN was able to keep the peace.

And many other peace keeping activities and human rights promotion.