for 1st world war: the murder of archduke franz fernidad on 28 june, 1914 of austria by a serbian fanatic. austria declared war on serbia which leads to world war 1

for world war 2: the german tanks drove into poland on 1st september, 1939. hitler invaded poland as punishment for refusing to handover the port of danzing to germany later leads to world war 2

the other main causes for world wars are aggressive nationalism, imperialism, secret alliances, militarism.
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