The assassination of Austrian archduke Franz Ferdinand and his wife by a Serbian national helped to ignite the conflict. Austria invaded Serbia. Germany, Austria-Hungary and Italy allied against Great Britain, France and Russia. The US entered the war in 1917 and helped Britain, France and Russia defeat the German alliance.
For more detail on the causes, battles and results of the "War to end all wars" see the link below. There are several excellent sites on WWI on the internet, check them out.
Further reading: https://en.wikipedia.org/wiki/World_War_I
Family connections: None that I'm aware of.