Winston Churchill was appointed First Lord of the Admiralty on September 3, 1939, the same day that the United Kingdom declared war on Nazi Germany. He succeeded Neville Chamberlain as Prime Minister on May 10, 1940, and held the position until July 26, 1945. During his tenure as Prime Minister, Churchill oversaw British involvement in the Allied war effort against the Axis powers. Churchill is widely regarded as a victorious wartime leader who played a crucial role in defending Europe's liberal democracy against the spread of fascism. However, some wartime events, such as the 1945 bombing of Dresden, have generated controversy. Churchill was the most important of the Allied leaders during the first half of World War 2. He rallied the nation in defiance of Hitler and streamlined planning and decision-making. His forceful personality was instrumental in cementing the 'Big Three' alliance with the United Kingdom's powerful allies, Russia and the United States. Churchill's speeches are among the most powerful ever delivered in the English language. His words were defiant, heroic, and human, lightened by flashes of humour. They reached out to everyone in the United Kingdom, across Nazi-occupied Europe, and throughout the world.

Joseph Stalin emerged as one of the most successful leaders of World War 2. Having attempted to form an alliance against Hitler with Western powers in mid-1939, Stalin reversed his position and made a pact with Hitler just before Germany invaded Poland, beginning World War 2. He believed this would give the Soviet Union time to prepare for war against Germany, while also allowing him to annex territory in Eastern Europe. The brief respite Stalin's bargaining gained him proved to be inadequate when Germany attacked the Soviet Union in June 1941, as his armies were torn apart by the experienced Germans. Stalin appointed himself as Supreme Commander-in-Chief and helped organize a successful counter-offensive when Moscow was threatened by the Germans in the winter of 1941. The Soviet Army, under Stalin's close direction, also won the Battle of Stalingrad in the winter of 1942-43. With the German defeat at the Battle of Kursk in the summer of 1943, the tide had turned against Germany, and the Red Army pushed them all the way back to Berlin, forcing Germany's surrender in May 1945. On the political front, Stalin participated in high-level meetings with his Western allies as a member of the 'Big Three' with Churchill and Roosevelt at Tehran in 1943, and then again at Yalta and Potsdam in 1945.

Adolf Hitler became the leader of Germany on January 30th, 1933, whereupon he immediately began the process of rearming Germany and reversing the injustices of the Versailles Treaty, re-occupying the Rhineland, and reintroducing conscription. Hitler was directly involved in the political successes that saw the Anschluss of Austria with Germany and the occupation and dismemberment of Czechoslovakia. By the summer of 1939, Hitler felt strong enough to embark on the military expansion of Germany, ordering the invasion of Poland on September 1st, 1939, which led to the beginning of World War 2. The first years of the war saw Germany achieve many victories, but by late 1942, having overreached himself, the tide of the war had started to turn against Germany. With America now in the war and Germany defeated at Stalingrad, he became increasingly isolated, paranoid, and irrational as the gravity of Germany's eventual defeat dawned on him. By the 30th of April 1945, and with the Red Army having fought to within metres of the Reichstag, he chose to commit suicide in his Berlin bunker rather than be captured.

Benito Mussolini was the dictator of Italy and the founder of fascism. During World War 2, he was the less dominant half of the Axis powers. Mussolini understood that peace was essential for Italy's well-being and that a long war might prove disastrous, but he was beset by concerns that the Germans might take all the glory and by not coming into the war early on their side, he would lose claim to conquered lands as Hitler's armies advanced. He therefore committed Italy to the war in June 1940, just before the French Armistice. Italian forces fared poorly from the outset, with significant defeats in North Africa and Greece and then in the Soviet Union. Mussolini had underestimated the extent of public support for his regime and for the war. When the Western Allies invaded Sicily in July 1943, the King arrested and imprisoned him with the intention of handing him over to the Allies, but he was rescued by German forces before this could happen. Mussolini was then made the head of a new Fascist state controlling the unoccupied parts of Northern Italy. In the turmoil of the last weeks of the war, he was captured by Communist partisans and executed.

Charles de Gaulle was a French general in the earlier stages of the war. However, when the French government signed an armistice with the Germans in June 1940, de Gaulle refused to accept it and escaped to London. With the backing and support of the British, he announced the formation of a French government in exile and became the leader of the Free French forces, which continued the fight alongside the British to liberate France from German occupation. After the liberation of Paris in August 1944, de Gaulle was given a hero's welcome in the French capital and became president of the provisional government, guiding France through the writing of the constitution on which the Fourth Republic was based. De Gaulle's leadership during World War 2 helped to restore democracy in France and laid the foundation for his later role as president of France's Fifth Republic.

Franklin D. Roosevelt was the 32nd President of the United States. Before America's entry into World War 2, he stood up strongly against the Axis powers whilst gradually preparing the country for war and aiding the British with weaponry. Once America entered the war, he supervised the mobilization of the American economy to support the war effort and implemented a Europe-first strategy. He met with Churchill in a number of wartime conferences, at which differences were settled and strategy decided. However, relations with the Soviet Union were more difficult, with the Soviet Union demanding large quantities of lend-lease supplies but seldom divulging its military plans or acting in coordination with its Western allies. Ultimately, Roosevelt believed that the maintenance of peace after the war depended on friendly relations with the Soviet Union. He worked hard to win the confidence of Joseph Stalin, whilst also working with the other Allied leaders to lay the groundwork for the United Nations and other post-war institutions.

Hideki Tojo was also a leading advocate of Japan's Tripartite Pact with Germany and Italy in 1940 and later served as the Prime Minister of Japan from 1941 to 1944. Tojo was known for being a hardworking and efficient bureaucrat, as well as one of the most aggressive militarists in the Japanese leadership. He led his country's war efforts after the attack on the U.S. military base at Pearl Harbor on December 7, 1941. Under his direction, Japan initially scored numerous victories throughout Southeast Asia and the western Pacific region. However, following a series of military reverses in the second half of 1942, Tojo assumed virtual dictatorial powers, taking over the post of Chief of the General Staff. Despite his efforts, Japan's losses mounted and its fragile industrial foundations collapsed. Tojo was removed as Chief of Staff and Prime Minister in July 1944. After the war, Tojo was tried and executed for his part in Japan's war crimes.