The 28th June marked the 100th anniversary of the assassination of Archduke Franz Ferdinand. This event is usually seen as the trigger that set off a chain of events that culminated in the First World War. However, the long term causes of that war are of course much more complex. You can find a good account of what happened that day in an article from the Daily Telegraph here. You might also like to watch this 5 minute Youtube clip with Dan Snow below.
Beyond the events of 28th June 1914, this History on the Net article is a nice introduction to the different causes, whilst in this excellent BBC magazine article historians discuss who should be blamed for war starting. Finally, watch the great AJP Taylor stressing the importance of the railway timetables.