World War I was referred to as "The Great War" because it was so big that a second one wasn't needed. The war started because Britain declared war on the Germanic Empire. This is because the Germanic Empire wanted to pass through Belgium, Britain's friend, to attack France. This was because France was also friends with Russia who was ready to attack Austria, and Austria was getting ready to attack Serbia because someone from Serbia shot the leader of Austria.
In 1917 CE, the Russian people were tired of Rasputin, a crazy-religious homeless guy determining the fate of their monarchy and had The Russian Revolution. This caused Russia to turn into The Soviet Union, who was the biggest communist country in history. At the end of the war, a bunch of countries created the League of Nations to prevent having another war. Also in 1917 CE, the Arabs Revolt and the Ottoman Empire was conquered by Britain. Britain also gave the Jews a place to live in Palestine. Then, Sykes and Picot decided to carve up the remains of the Ottoman Empire.
Germany got rekked by the Treaty of Versailles. They lost territory, demilitarized the Rhinelands, reduced their army to 100,000 men, couldn't have an air force, and had to pay the allies hundreds of billions of dollars. Then, the Saudis conquer Arabia and make Saudi Arabia, thus making the entire Arabian Peninsula controlled. After that, the Roaring Twenties happened in America caused by things such as the phone, the car, jazz, the radio, movies, and other modernizations. Just when everything was going so well, The Great Depression happened around the world.
In 1933 CE, Adolf Hitler came to power in Germany to solve all the problems they were having. and was mad at the Jews for hindering them during World War I. Then, Japan created the Japanese Empire, colonizing Southeast Asia again. Hitler wanted more living space for his people so he invaded Austria, Czechoslovakia, Belgium, the Netherlands, France, Norway, Hungary, Romania, Yugoslavia, Bulgaria, and Greece. As seen in the map above, Hitler was out of control so The Allies scared him back into Germany and tried to tell him that killing all the Jews was a bad idea. The United States was mad at Japan during the war because they bombed Pearl Harbour. The United States got so mad that they bombed two Japanese cities in 1945, making Japan surrender for good.
After the war, the United Nations wanted to have some world peace. Then, Mahatma Gandhi helped India gain independence from Britain. Back in Arabia, the United Nations divided up the Holy Land between the Jews and the Arabs to make them stop fighting. This only made things more complicated. In 1949 CE, a communist government took over China.