Question

why world war and world war 2 happen

Answer 1

The direct cause of WWI was the assassination of Archduke Franz Ferdinand at Sarajevo on 28 June 1914. However historians feel that a number of factors contributed to the rivalry between the Great powers that allowed war on such a wide-scale to break out.
It involved many of the world's countries. The Second World War was started by Germany in an unprovoked attack on Poland. Britain and France declared war on Germany after Hitler had refused to abort his invasion of Poland.

Answer 2

The first world war started after the assassination of Archduke Franz Ferdinand of the Austro-Hunagarian empire, by Serbian nationalists. This sparked a chain of events, with Austri-Hungary declaring war with Serbia. Serbia was allies with Russia and France, so they declared war on Austria-Hungary. Then Germany was allies with Austria-Hungary, and so Germany declared war on Russia and France. Then France was allies with Britain, so war was declared between them. On top of this, France and Germany had just been at war not long ago, fighting the Franco-Prussian war. The first world war was merely an excuse to take more French territory for the Germans.

The second world war started when Nazi Germany invaded Poland, after British and French attempts to appease the Nazi leader, Adolf Hitler, failed.