World War II was not only marked by the bloodshed and sheer number of lives lost in battle, but was also defined by the complex political decision made among the leaders of the world powers. In the United States, the war presented President Franklin Delano Roosevelt with many political and military challenges; and the decisions he made created ripples that affected the world for decades after World War II ended. The Cold War began after the end of World War II and was defined by the rivalry between the United States and the Soviet Union and their allies1. Roosevelt’s military and political decisions during World War II led to distrust and animosity between the Soviet Union and the United States along with other powers in the west that led to the beginning of and persisted throughout the Cold War.Along with Winston Churchill of Britain, Roosevelt had different military visions from Stalin during the war. In 1943, Roosevelt and Churchill decided to launch an invasion of Italy—a decision which Stalin heavily disagreed with2. Stalin had been pushing for the opening of a second front in France2 in order to alleviate some of the pressure on the Red Army and force German troops to move westward. Stalin feelings of being slighted as his troops were facing the brunt of the German Army, while Churchill and Roosevelt were hesitant to launch an assault in France and face heavy casualties caused further splits in the Soviet-western Allies relationships. At the Tehran conference in 1943, Roosevelt and Churchill promised Stalin that the western Allies would invade France in 1944 and Operation Overlord, which outlined an Allied campaign in northern France5, was established. However, Stalin was still enraged that the Soviet army would only gain relief so late into the war. These rifts in the relationships between the Soviet Union and the United States and other western Allies that widened would contribute to the start of the Cold War. In 1945, the Big Three—the U.S., Britain, and the Soviet Union—met at the Yalta conference to plan for the future of the world after World War II4. Churchill, Stalin, and Roosevelt disagreed on many goals and ideas, which resulted in much tension among the powers that helped pave the road to the post-war Cold War. Stalin wanted the other powers to financially aid Russia after the war, however Roosevelt and Churchill did not agree to any definite form of economic aid for the Soviets8, which unsettled Stalin. Roosevelt also sought to establish the United Nations, in which Stalin pushed for the representation of all sixteen Soviet republics in the General Assembly. However, the Soviet Union ultimately only gained three seats in the UN for Belarus and Ukraine in addition to the Soviet Union as a whole8, further adding to Stalin’s dissatisfaction with the agreements. On the issue of post-war Germany, Stalin expressed his desire to weaken Germany and dismantle heavy industry in the nation. Roosevelt, on the other hand, disagreed and felt that Germany should be reunited and reconstructed under Allied supervision6. Eventually, the agreed upon compromise outlined that Germany would be divided into zones of occupation under the administrations of the U.S., Britain, the Soviet Union, and France7. The compromises made at Yalta left Roosevelt, Stalin, and Churchill unsatisfied, and disagreements specifically between Roosevelt and Stalin’s viewpoints rocked the relationship between the U.S. and the Soviet Union that would eventually sour in the Cold War.In addition, at Yalta, Roosevelt aimed to have democratic governments across liberated Europe, however there was dissension between his views and Stalin’s. Stalin was in favor of establishing communist regimes in liberated countries and had already implemented Soviet-controlled non-democratic governments in Poland and other nations in Eastern Europe such as Bulgaria, Romania, and Hungary before the end of the war6. Since Stalin was reluctant to relinquish the power he held over these nations, Roosevelt was forced to make a compromise that was widely condemned by the American public, who believed that Roosevelt was allowing Stalin’s power and communist influence in Eastern Europe to go unchecked. The Declaration of Liberated Europe outlined that the United States, Britain, and the Soviet Union would assist liberated European states in forming governments that broadly resembled democracies and establish free elections as early as possible4. Without indicating a concrete time frame for these institutions to be put in place, this agreement allowed Stalin to continue to impose his Red Army and communist influences on Eastern Europe, violating the pledge made in the Declaration of Liberated Europe. In Poland, specifically, the Soviet Union’s communist control did not allow free elections to take place for another forty years6,8. Roosevelt’s concessions to Stalin’s demands and the Soviet Union’s disregard of the goals expressed in the Declaration made at the Yalta Conference was heavily criticized by politicians and citizens alike in the United States, which further tensed the relationship between the Soviet Union and the U.S.Another impactful decision Roosevelt made during World War II was the establishment of the Manhattan Project in 19416, which led to the development of the first atomic bomb used in war. The U.S.’s secretive experimentation with nuclear weapons, eventually led to even more anger and distrust from the Soviet Union when the project was exposed in 1945 when the atomic bomb was dropped in Hiroshima, Japan. Even though Stalin was aware of the U.S.’s nuclear projects from Soviet Intelligence3, the fact that Roosevelt did not inform him of this weapon of mass destruction or the U.S.’s plan to release it onto Japan angered Stalin. This led to even more tension at the Potsdam conference in which President Truman formally informed Stalin about the successful detonation of the atomic bomb on Hiroshima and the leaders continued to disagree about war reparations from Germany and Soviet control in Poland3, further amplifying the animosity between the Soviet Union and the United States and other western powers.Additionally, back at Yalta, Roosevelt did not know if the plan to detonate the atomic bomb in Japan would work in bringing down the Japanese troops, so he secured the help of the Soviet Union in the war against Japan4, making concessions that left many Americans unsatisfied. Roosevelt conceded to Stalin’s conditions—the southern part of Sakhalin and its islands will be returned to the U.S.S.R., Soviet interests in the Dairen commercial port will be protected, Port Arthur will be restored as a USSR naval base, the Chinese-Eastern Railroad and South Manchurian Railroad will be operated by a joint Soviet-Chinese company, and the Kuril Islands will be given to the Soviet Union4—in exchange for military assistance in the war against Japan. The dropping of the atomic bomb on Hiroshima was successful in subduing the Japanese, so the previously agreed upon Soviet assistance in the war between the U.S. and Japan from the Yalta conference was no longer necessary in order to secure an American victory against Japan. However, the Soviet Union still declared war on Japan and Stalin was able to gain control over more territories in East Asia and spread communist influences, which unsettled many of the citizens of democratic nations in the west, especially in the U.S. This fear of communism and hostility of the American people towards the Soviet Union would also contribute greatly to the beginning of the Cold War.While the United States and the Soviet Union were allies during World War II, Roosevelt and Stalin’s visions were far from the same. Roosevelt’s disagreements with Stalin over military and political decisions during the war and regarding the post-war world led to fractures in Soviet—U.S. relations and shaped the two countries’ rivalry that would lead to the Cold War.