World War II
World War I had greatly changed the way of diplomacy and politics in Asia, Europe, and Africa with the defeat of the Central Powers.  The empires that had sided with the Central Powers were destroyed.  Even the Russian Empire, which was on the Allied side, broke up and became the USSR (Soviet Union) after the Russian Civil War.  The war also changed the borders in Eastern Europe, with many new countries being born.  The war led to strong irredentism and revanchism, which was especially strong in Germany since it had been forced to sign the Treaty of Versailles.  The Germans also lost 13% of their homeland area and all of their colonies, and they had to pay back a very large sum of money to the Allies.  The size of their army and navy was also limited, and its air force was banned.