Alliance systems that were in place during World War I included the Triple Entente (Britain, Russia, France), the Triple Alliance (Italy, Germany, Austria-Hungary), the Russia-Serbia Treaty and the Britain-Japan Treaty. These alliances contributed to the enormous scale of WWI.
World War I began in 1914 when Austria-Hungary declared war on Serbia after the June 28 assassination of Archduke Ferdinand by Gavrilo Princip, a Serbian. Due to the alliance systems in place at the time, other countries joined in based on their treaties with other countries. For example, Russia aided Serbia, which caused France to enter based on their alliance with Russia. On the other side, Germany aided Austria-Hungary due to their alliance.