An alliance is an agreement made by two or more parties to secure common goals and to defend the other party in case of war. They were formed to prevent war and to maintain a balance of power. But one event could lead up to a huge conflict.
The alliances are the main reason behind World War 1 because they turned what was meant to be a brief war between accuser and accused into a world war. How the alliances turned a brief war into a world war is a classic case of one thing led to another.