The Treaty of Versailles was too harsh on Germany: taking away land, money (reparations) and weapons, this made Germany angry leading to the rise of Hitler
Germany, Italy and Japan were lead by rulers who wanted land and power, they rejected international policies and took over other countries
Appeasement was used by Britain and France for Hitlers early acts of aggression (eg remilitarisation of Rhineland) to avoid conflict, however appeasement only encouraged Hitler to keep commiting aggressive action
Failures of the League of Nations (eg Abyssinian crisis) caused tensions to rise
GreatDepression made people poor and in need so they were more likely to listen to bad leaders who promised them a better life
Hitler was building up the German military, violated the terms of the Treaty of Versailles, this caused countries to feel fear and insecurity
Hitler invasion of Poland in 1939 caused Britain and France to declare war on Germany