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

The Kingdom Of Italy was part of the Central Powers, but then changed sides.
As a part of the Triple Alliance,their allegiance was intended to counterbalance the Triple Entente allegiance of France, UK, and Russia. However, at hostilities, the Kingdom Of Italy did not join the fighting and instead joined the allied side (France, UK, and Russia) side in May 1915.
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Answer 2

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