How did the alliance system cause war?

Having military allies means that if your allies go to war, you go to war as well, to help them fight. So small wars become much larger wars. Also, systems of alliances make nations over confident. One of the causes of WW I was the belief by both sides of the war that their military alliance was so large and so powerful that it could not be defeated. People are always more willing to go to war when they expect to win. Without their allies, they might have been more hesitant to take such risks.