The international peacekeeping organization formed directly after world war ii was called

 THE UNITED NATIONS

Further details/context:

A conference of delegates from 39 nations was held at Dumberton Oaks, a historic estate in Washington, DC, as World War II was still being fought.  The official name of the gathering, which took place from August 21 to October 7, 1944, was the Washington Conversations on International Peace and Security Organization.

The ultimate result of this conference, following the war, was the establishment of The United Nations.  The UN Charter, signed in 1945, lists the purposes of the organization in Chapter I, Article 1, as follows:

The Purposes of the United Nations are:

To maintain international peace and security, and to that end: to take effective collective measures for the prevention and removal of threats to the peace, and for the suppression of acts of aggression or other breaches of the peace, and to bring about by peaceful means, and in conformity with the principles of justice and international law, adjustment or settlement of international disputes or situations which might lead to a breach of the peace;To develop friendly relations among nations based on respect for the principle of equal rights and self-determination of peoples, and to take other appropriate measures to strengthen universal peace;To achieve international co-operation in solving international problems of an economic, social, cultural, or humanitarian character, and in promoting and encouraging respect for human rights and for fundamental freedoms for all without distinction as to race, sex, language, or religion; andTo be a centre for harmonizing the actions of nations in the attainment of these common ends.  

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