What started world war 1

There were a lot of reasons.

Political reasons:

England and Germany wanted leading position in industry.

France wanted to reconquer Alsace and Lorraine. Those two regions had a lot of iron and coal.

Russia was interested about the Balkan Peninsula and didn't want Austria to expand there.

Serbia wanted to found a new State of only bordering States.

Italy wanted to reconquer regions that were dominated by Austria.

Another cause would be the race among leading countries in industries.

Another one would be the positive judgement of the War, which according to people, would have changed economical and social situation.

But, to make this happen, on 28th June 1914 the Archduke Franz Ferdinand of Austria was killed in Sarajevo (Bosnia) by a Serbian nationalist.