In which year did the Second World War end?

The war ended in 1945, but in two stages, which is why the date is sometimes muddled. Germany surrendered unconditionally in early May, after Soviet forces took Berlin and Hitler killed himself in his bunker, and the surrender was signed twice, at Reims and again in Berlin, to satisfy Soviet insistence on a ceremony on their terms. That is VE Day. The war against Japan ran on for another three months across the Pacific, ending after the atomic bombings of Hiroshima and Nagasaki and the Soviet declaration of war on Japan, with the surrender announced in mid-August and formally signed aboard an American battleship in Tokyo Bay in early September. The gap explains why different countries mark different anniversaries. The war had begun in Europe in 1939 with the invasion of Poland, though fighting in Asia had started earlier, and it left the world divided between two rival blocs.

The United Nations was founded in 1945, drafted while the fighting was still going on.