Final answer:
The main alliances that triggered World War I were the Triple Entente and the Central Powers. Their complex network of treaties and mutual protection obligations escalated a regional dispute into a global conflict, drawing countries like Serbia, Belgium, and later Italy into the war.
Step-by-step explanation:
The major alliances in World War I were the Triple Entente, consisting of France, Great Britain, and Russia, and the Central Powers, including Germany, Austria-Hungary, and the Ottoman Empire. These alliances were responsible for the escalation of the war, drawing multiple countries into the conflict.
For instance, Serbia's alliance with Russia pulled it into the war after the assassination of Archduke Franz Ferdinand, which prompted Austria-Hungary to declare war on Serbia. Conversely, Italy, initially aligned with the Central Powers, switched sides to join the Triple Entente in 1915.
The entangling alliances meant that when Austria-Hungary, backed by Germany, declared war on Serbia, Russia was obliged to come to Serbia's aid, leading to a chain reaction of mobilizations and declarations of war. The invasion of Belgium by Germany led Britain to declare war on Germany, due to Britain's traditional alliance and protector status of Belgium. This expansive web of alliances turned a regional conflict into a global war.