Italy was the European country that entered WWI as part of the Central powers but later switched sides to the Allies in 1915.
Explanation:
The Kingdom of Italy was an ally of the Central Powers at the eruption of warfare One in August 1914. As a member of "Triple Alliance" their allegiance was meant to counter-balance the Triple Entente allegiance of France, UK and Russia. However, at the eruption of hostilities, the dominion of Italy didn't take part fighting and instead joined the allied (France, Russia, United Kingdom) aspect in might 1915.
