President Franklin D. Roosevelt of the United States
President Roosevelt was the thirty-second president of the United States, elected due to his promises to bring the country out of depression. Being the only President to be elected more than two terms, he led America out of depression and through World War Two. For much of the war, he kept to the Neutrality Acts meaning that he didn't aid either side of the war. Most actions made by Hitler prior to the war were for the most part ignored by Roosevelt and the political authority of the US as they didn't directly affect the US (for instance, the US had no binding to the Treaty of Versailles). Eventually as the war progressed worse for the Allies, he allowed for the aiding of Britain with the Lend Lease Act and Atlantic Charter which brought the US closer to war. Thus, Roosevelt's role in the cause of World War Two wasn't too large until during the war when he aided the Allies and froze Japanese assets.
U.S. Secretary of State Cordell Hull
The main man that represented relations between states for President Roosevelt. He served to demand indemnity from Japan after the Japanese attacked US ships on the Yangtze and reject the Japanese propositions. He believed that strife going on in Europe should remain in Europe thus making the US neutral; this philosophy was seen by the President as well.
Chancellor Adolf Hitler of Germany
Adolf Hitler is the man most responsible for the beginnings of World War II. As a child and young adult, he had a particularly poor life being pushed against his dreams of art and eventually being revoked as a failure in that field. Having served in World War One, Hitler had blamed the Jews and communists for Germany's defeat and eventual suffering. He sought to change Germany and get it to rise to power (via his rule) and had written Mein Kampf. Due to his desires, he became a part of the Nationalsozialistische Deutsche Arbeiterpartei (Nazi party). Because of the depression, Hitler's promises of German glory and rise out of such stirred hopes in the people. After Hitler was appointed chancellor in 1933 and eventually the Commander of the Army, he began his plan to make Germany a world power by taking other lands that had Germanic people. This included Austria and the Sudetenland of Czechoslovakia, but he took all of Czechoslovakia anyway. He defied the Treaty of Versailles in every possible way, and the other countries (namingly Britain and France) did nothing to stop him because they feared that if they had tried, war would have broken out. As it happens, war occurred anyway with Hitler's invasion on Poland in 1939.
President Benito Mussolini of Italy
Mussolini became the dictator of Italy when he marched against Rome in 1922. Mussolini, member of the Fascist party, was against communism and fervent on the idea of getting Italy to rise to power and escape depression. Mussolini, actually, influenced Hitler. However, as Mussolini had taken power rather than being voted into it, he did not have as much people support as Hitler did. With this, most of his attempts in capturing land failed with Albania and Ethiopia. His conquest for Ethiopia especially provided the prospect that capturing land may become an issue (which became evermore apparent with Hitler's conquests).
General Hideki Tojo of Japan
Before World War Two and his time as the Prime Minister of Japan, Tojo was the general of the Japanese army. Japan due to its new dictator-like government had a more aggressive foreign policy, and thus the Japanese had a certain admiration for the dictators of Europe (which included Hitler and Mussolini). He was responsible for the Japanese attacks on areas such as Manchuria and the Viet Nam area, and he served as a major figure (even politically) during the war with China. Overall, his actions would lead the United States into World War Two as he lead the war on the Asian front.
Generalissimo Francisco Franco of Spain
Francisco Franco was a prominent figure in Spain during and after the Spanish Civil War. In the later 1930s, civil war had erupted in Spain between the Nationalists and the Republicans. In this civil war, Francisco Franco became the Generalissimo and through his tactical skill and aid from Germany and Italy, he defeated the Republicans and won the war. With this he became the dictator of Spain, and the countries of the time simply accepted this. Due to the loss of democracy in Spain, this only caused the future Ally forces to resent Germany and Italy for aiding the dictator, and the tension would only tear the two sides apart making more resent.
Prime Minister Winston Churchill of the United Kingdom
He succeeded Neville in 1940 after Neville failed in the aiding of the Norwegians when Germany attacked Norway. Churchill was not going to back down from Germany and governed Britain for the duration of the war. Churchill did not even back down when France fell, showing his determination. Because of Churchill's determination, Hitler could not get Britain out of the war like France causing him to go ahead with his plans to invade the USSR. This greatly aided the Ally cause.