After World War II 12 countries including the United States joined?

The correct answer is: NATO

North Atlantic Treaty Organization

It was founded in April 4th, 1949. Its original members were Belgium, Canada, Denmark, France, Iceland, Italy, Luxembourg, the Netherlands, Norway, Portugal, the United Kingdom and the United States.

It was created to serve to different purposes: deterring Soviet expansionism, to avoid the revival of nationalism militarism in Europe (like the Nazi Germany or the Mussolini Italy) and encouraging European political integration. Lets remeber that Europe was devastated after the war and the URSS took control over some territories, these countries wanted to preserve democracy and stop comunism from expanding to all the continent. They did several effords in order to achieve this, for example the Marshall Plan. The creation of the NATO fits in this description too.