Question

Which European country entered World War I as part of the Central Powers but later switched sides to the Allies in 1915?

Answer 1

Italy,
Italy was part of the so called "Central Powers" but didn't enter the war (beside Germany and Austria-Hungary) remaining neutral (the clause of the treaty called for support against an aggressor not to be the aggressor) up to 1915.

Answer 2

Italy was the European country which entered World War I as part of Central Powers but later switched sides to the Allies in 1915.

The Italy entered into World War I as part of Central Powers consisting of Germany, Austria-Hungry and others. It later switched to the Allies in 1915 as  She had become convinced that the alliance with the Central Powers would not gain Italy the territories she wanted as they were under the possession of Austria-the old enemy of Italy.