At Tehran, held in the winter of 1943, the Allied leaders agreed that a second front should be opened against Nazi Germany. Stalin, whose Soviet Union was bearing most of the brunt of the war against Germany, argued for the immediate commencement of a second front, and it was decided that operations would commence to that end in 1944. Stalin also secured tentative support for his intention to exercise influence in Poland, including redrawing its borders, after the war.
At Yalta, held in 1944, the Allies agreed that Germany must surrender unconditionally, and that that nation would be split into occupation zones between the USSR, Great Britain, France and the USA. Also, Stalin agreed to democratic elections in Poland, but stipulated that the Polish government would be friendly to communism. Also, Stalin agreed that after Germany surrendered, the USSR would enter the war against Japan.
At the Potsdam Conference, which was held after Germany's surrender, the Allies agreed to the partition of Germany. The United States, with the support of its allies, also issued a declaration demanding the unconditional and immediate surrender of Japan. However, the conference was notable more for the tensions that began to develop there than for any agreements. Harry Truman, now president of the United States, was incensed that Stalin had reneged on his promise to hold free elections in Poland, and further tensions were developing over the occupation of Germany and the future of postwar Europe. For these reasons, Potsdam (though some point to Yalta) is often viewed as the starting point for the Cold War.