As a result, the purpose of this thesis is to discuss how crucial the participation of the United States in the Second World War was, and its main goal is to elaborate on how its entry changed the outcome of the entire war. Starting from the Battle of Pearl Harbor (after which the U.S. joined the war) and ending the thesis with the discussion of dropping the atomic bombs and the signing of the peace treaties (which ended the First World War). Its narrative will be unfolded through three chapters, giving the reader a relatively detailed picture about the events. The first chapter is an introductory part, which provides insights into the Interbellum Era (or Interwar Period) and the circumstances of the beginning of the Second World War. The second chapter is based around the overview of why and how the United States joined the war (as a result of Pearl Harbor) and the most important battles at the Pacific Theater are introduced, explained with a focus on why they are important regarding the outcome of the war. And this will start the third and closing chapter, which delves into the deployment of atomic bombs, the ending of the Second World War (including the Peace Treaties). Together, these chapters provide a narrative that reflects on how the United States’ entry into the war itself changed the outcome of the war.