What caused World War 1 and World War 2?

Answer 1

World War I started as a long chain of events that began with a small assassination. Franz Ferdinand, the heir to the Austrian throne, was murdered by a Serbian terrorist group called the Black Hand, while he was touring Serbia. The Austrians pointed the finger at the Serbian government (which was partially correct - really, Serbia could have prevented it if they wanted to) and threatened them with war. Serbia sought the support of Russia, who agreed to fight if Austria declared war. Serbia refused the ultimatum, and the two superpowers declared war.

Germany, sensing a huge European conflict, declared war on France as a pre-emptive move to avoid a war on two fronts (which ended up happening anyway). To outflank the French army, Germany invaded Belgium on route to northern France. As this was a German violation of Belgium's neutrality, Britain declared war on Germany to protect Belgium. Hence, all of Europe's major powers were at war - Britain, France & Russia vs Germany, Austria-Hungary and Turkey. Italy, having previously declared neutrality, joined the allies' side in 1915.

In World War II, Germany, under Hitler, began taking small surrounding territories that it had lost in the aftermath of World War I. After Hitler invaded Poland, the Allies decided enough was enough, and declared war on Germany. Germany then took France extremely quickly, and took most of Europe that wasn't allied. Soon, only Britain was left standing. After getting nowhere in Britain for months, Germany decided to invade Russia, who actually had a peace treaty with Germany.
The assassination of Archduke Franz Ferdinand sparked WW1, WW1 caused WW2 because Germany was treated very harshly in the Treaty of Versailles and thus, in a state of weakened economic power (also caused by the Great Depression), radical leaders such as Hitler were able to rise to power. The immediate cause of WW2 was Germany's invasion of Poland