Why did World War 1 erupt in eruope?

A minor war broke out between the two fairly small and not powerful countries of Austria Hungary and Serbia.

A very powerful Germany promised to back up Austria Hungary, a very powerful Russia promised to back up Serbia.

Here is where it gets complicated, Russia saw this and was now concerned about a war with Germany and moved their troops to be ready to attack Germany.

Germany saw this and was concerned if they were attacked in the east by Russia (they were a few days later), and had all their troops in the east, then France would attack Germany on their West and take back some area they had been fighting with France over for some time.

So... history says Germany attacked France (and Belgium) first, figuring they would be easy to defeat, then they would concentrate their efforts on a much more powerful Russia attacking them on their eastern front.

But England got involved and very quickly started to help France against the German attacks.

Thereby the most powerful countries in the world very quickly got locked into a very deadly and equally balanced conflict, almost before anyone realized what was happening. All this was at a stalemate for 4 years or so until the USA joined the war and helped defeat Germany.

So combine all those factors and add in the ineffective and slow communication technologies of the day, and a general lack of understanding how horrible modern technology could make warfare, nobody realized how serious all that was becoming before it was WAY too late.