World War I, also known as the Great War, began in August 1914, with the outbreak of war between Germany and the Austro-Hungarian Empire. The Allies of Britain and France declared war on Germany, and the Central Powers, including Russia, Austria-Hungary, Romania, and Japan, declared war on Germany.

After the outbreak of war, a League of Nations was established to prevent future conflicts. It was in this league that the 1919 Treaty of Versailles was negotiated, recognizing the responsibility of Germany and limiting its army.

After the Treaty of Versailles, Adolf Hitler, the absolute ruler of Germany, became a threat to the peace. He began to incite war against countries across Europe. In the end, the Allies of Britain and France and the United States swept aside the German Empire.

The conflict was fought on two fronts, with the Allies of the British, French, and Russian empires fighting on the Western Front, and the Allies of Japan, China, and the Soviet Union on the Eastern Front. The Central Powers, however, continued to fight.

A large German invasion of Belgium and northern France began the war. The Allies, including France and Britain, were forced to counterattack in July and August.

Germany and her allies attacked the Soviet Union in December. However, a massive Soviet encirclement stopped their advance. They were defeated at Stalingrad, and the Soviet Union regained its strength.

Italy entered the war in June 1940, and was followed by a series of German offensives in North Africa. Italy, however, was unable to defeat the Germans, and the Allies were able to keep them from taking over Italy.