There were several causes of World War I. One cause was the system of alliances that existed before the war began. There were two alliance systems before the start of World War I. The Triple Alliance included Germany, Austria-Hungary, and Italy. The Triple Entente included France, Great Britain, and Russia. In an alliance, countries agree to help each other if they are attacked.
The danger of the alliance system is that if a member of one alliance declares war on a member of the other alliance, the conflict could quickly escalate if other members of each alliance join the war. A two-country conflict can quickly involve many nations as a result of the alliances that had been formed. This is what happened in World War I.
In World War I, Austria-Hungary declared war on Serbia. Serbia and Russia were allies. Russia came to Serbia’s defense and declared war on Austria-Hungary. Thus, a member of each alliance had declared war on each other. After Russia declared war on Austria-Hungary, Germany declared war on Russia. France declared war on Germany, and Germany declared war on France. Eventually, Great Britain joined the conflict. Thus, the system of alliances placed a significant role in the start of World War I.