How did international relations develop during the Second World War?
International relations developed in several crucial ways during World War II. The first is that continental Europe, by virtue of the destruction of Germany as a viable political force, ceased to exert much influence on geopolitics. The world, in a word, became bipolar, with the United States and the Soviet Union emerging as "superpowers." This is not to downplay the importance of Europe, which after all was the center of the early Cold War, but European nations, particularly Germany and France, and to a lesser extent Great Britain, were no longer the global powers they had been in the early twentieth century.
Another crucial change was the turn to global institutions as a means of maintaining peace. The United Nations, founded in the waning days of World War II, was viewed as a forum to resolve disputes before war broke out. It became, in many ways, a battleground for the United States and Soviet Union, but it also represented a faith in institutions shared by many Allied leaders, especially in the United States.
The war also saw the rise of nationalism among many colonial peoples. Many of the European colonies, especially in North Africa, Southeast Asia, and the Pacific, became battlegrounds in the war. Nationalist movements that formed in resistance to Japanese and German occupiers became advocates for independence after the war.
Finally, the war saw two major diplomatic and strategic developments that altered the course of history. One was the drifting apart of the Soviet Union and the United States. Never the coziest of allies (though more so than we might imagine today), the two sides fell out over the fate of eastern Europe, and their disagreements hardened into Cold War hostility. The Cold War was "fought" in the shadow of another major World War II development, i.e. the atom bomb.