Answer:
WW1
The war started mainly because of four aspects: Militarism, Alliances, Imperialism and Nationalism. This is because big armies become potential threats to other countries, other countries started forcing alliances in order to secure land.
WW2
The Treaty of Versailles and the German desire for revenge.
Economic downturns.
Nazi ideology and Lebensraum.
The rise of extremism and the forging of alliances.
The failure of appeasement.