What caused WWI?

Wow, joxy, that is really a question and a half. Whole
books have been written on the subject, so I can't do much more here than give you a few
general causes.

1. The assassination of the
   Austrian Archduke Franz Ferdinand.  This ultimately led to the demands Austria made on
   Serbia that, when not totally accepted, triggered an invasion (and started
   WWI.)


2. The system of alliances, by which Europe was an
   interlocked net of countries that "had each-other's back."  It went a little something
   like this: Austria declared war on Serbia / Russia and France get ready to declare war
   on Austria / Germany declares war on Russia and France first (to get a jump on them and
   help their buddies the Austrians) / the UK declares war on Germany (because Germany, to
   get at France, rolls through neutral Belgium and won't get out) / Turkey jumps in on
   Germany's side / Italy (though allied with Germany) decides to backstab it by joining
   the "allies" / and finally the Americans stomp there way in to seal Germany's
   fate.


3. New technology and a massive arms build up that
   gave nations an itchy trigger finger.


4. The notion that
   wars were often fought, won, and lost quickly and without significant bloodshed.
   

There are a lot of other, more complex
reasons, but these are really the big ones.  Basically, everyone thought they could win
quickly and everyone had a lot of big friends to bring to the party.  In the end, there
was a whole lot of dying, the re-working of Europe's map, and a bitterness that let to
World War I part 2 about 20 years later.