The definition of an alliance system is a formal agreement or treaty between two or more nations to cooperate for specific purposes. An alliance system can also be defined as an agreement between individuals, families or corporations. However, the term alliance system is most often used in reference to a historical event when an agreement was reached to prevent the outbreak of war or to define territorial borders.
The outbreak of World War I best exemplifies how an alliance system works. The original cause of this war was the assassination of Archduke Franz Ferdinand, an heir to the Austrian throne. Because a Serbian national was responsible for the assassination, Serbia and Austria-Hungary declared war on each other.
The fact that this event caused the involvement of the United States and European nations is due to the alliance systems that were formed at that time. For example, Russia was allied with Serbia because of an existing treaty, so this nation became involved in the war. Likewise, a treaty bound Germany as an ally to Austria, so this nation became involved as well.
Furthermore, France was allied to Russia and Britain was allied to France, so these nations became involved as well. Ultimately, alliance systems led to a world war because all of the major nations had treaties with one another that determined their alliance in the event that war broke out.