What are the United Nations's main contributions towards world peace?

This is a question whose answer is debated among scholars
of international relations.

In theory, the United Nations
was supposed to prevent wars from occurring.  It was supposed to do this by providing a
place for countries to talk to one another rather than fighting.  It was also supposed
to do this through "collective security," which is the idea that all countries will come
to the defense of a country that is attacked.  Clearly, the UN has not been able to
prevent all wars since there have been many wars since the UN was
created.

So what does the UN do to contribute to peace? 
Some international relations scholars say that the UN contributes to peace by setting up
systems in which countries have a great deal of contact with one another.  The various
UN organizations allow countries to work together towards things like eradicating
various diseases or trying to protect the environment.  Scholars argue that all of these
interactions help to prevent conflict.  The interactions make the countries less likely
to fight one another because they are used to
cooperating.

In this way, scholars think that the UN has
contributed to world peace in an indirect way.  It has helped countries become more used
to interacting peacefully and has thereby stopped them from fighting.  Clearly, this has
not been successful in all cases, but it is said to help make the world more
peaceful.