Important events: World War I: Archduke Ferdinand and his wife, the Austrian Crown Prince, were assassinated by Serbian patriotic youth Princip with pistols; Battle of isle of man; Battle of somme; Battle of Verdun; Russian summer offensive; Jutland naval battle broke out by the main naval forces of Britain and Germany; The United States entered the war; Russian October Revolution; At that time, the Beiyang government of China joined the Allies to fight; Japan also participated in the war; Paris Peace Conference; The May 4th Movement in China.
World War II: Germany and the Soviet Union signed a non-aggression treaty; The Germans attacked Poland, and the Soviet Union expanded its territory to the west to establish the Eastern Front. Sufen War; The Germans attacked northern Europe, and western Europe and France perished; Battle of Britain; Soviet-German War (Moscow Campaign, Stalingrad Campaign, Kursk Campaign; Ten Soviet assaults); Alaman triumphed; The Japanese naval joint fleet attacked Pearl Harbor with the support of the air force; Battle of midway; Soviet troops conquered Berlin; The United States dropped two atomic bombs on Hiroshima and Nagasaki, Japan; Nuremberg and Tokyo trials; Cairo, Tehran, Yalta and Potsdam Conferences
The reasons for the outbreak are different. There are more countries participating in the war, more advanced weapons, a qualitative leap in operational concepts, and the post-war world political structure is also different.
All the above are my humble opinions, and criticism is welcome. Personal views are for reference only, at your own risk.