What was the immediate cause for outbreak of the World War I?

What was the immediate cause for outbreak of the World War I?

پہلی جنگ عظیم شروع ہونے کی فوری وجہ کیا تھی؟
Explanation

The assassination of Archduke Franz Ferdinand was the immediate cause of the war

between two great powers—Russia and the Habsburg Austro-Hungarian Empire

It was a global war originating in Europe

It lasted from 28 July 1914 to 11 November 1918.

Chlorine was the First gas used during World War I 

This gas was used at Ypres in April 1915