Final answer:
Italy was the country that remained neutral at the outset of World War I and joined the Allies in 1915 to gain territorial advantages over Austria-Hungary.
Step-by-step explanation:
The country that was neutral at the beginning of World War I and then joined the Allies in 1915 is Italy. Italy had initially been a member of the Triple Alliance with Germany and Austria-Hungary but chose neutrality when the war erupted. Italy's neutrality proved short-lived due to the opportunity to acquire territory from Austria-Hungary, and thus Italy entered the war on the side of the Allied Powers after securing support through a secret agreement. The shift was a strategic decision influenced by territorial disputes and a general dislike of Austrian rule among Italians.