What organization was set up to prevent other world wars?
There was an organization called the League of Nations, (or "LON"
in short) which was formed after World War 1. Its aim was to
prevent wars from taking place and world-wide disarmament. Sadly,
the main powers like Britain and France were self-minded, and
dealing with Great Powers like Japan, Germany, and Italy were
inefficient. Several small countries had solved their disputes,
however, because they feared trade embargos with these world powers
would adversely affect their economy.
When Communists came into power in the USSR (in 1917), the League
of Nations banned the USSR, as they feared the spread of Communism.
Although the United States started the idea of the LON, the USA did
not join because of their isolationism policy.
These inefficiencies of the League of Nations led to outbreak of
World War 2. Fortunately, in 1946 the League of Nations volunteered
to dissolve themselves because the United Nations (or "UN") had
formed in 1945. It has gone from 51 founding members to 192 current
members. Today, just about every country - including Israel, North
Korea, Iran, and Iraq has joined the UN. Currently (as of November
2009), there are only three countries not a part of the United
Nations: Kosovo, who gained independence in February of 2008 (but
is not internationaly recognised), the Vatican City, who opted not
to join, and Taiwan.
The issue with Taiwan is a little complicated, but the main problem
is representing both the People's Republic of China and Taiwan at
the same time.