What role and side did Italy fight on during World War 1? Why did they change sides?

Write this in your own words.

1: Before WW1 Italy was part of an alliance with Germany and Austria-Hungary, yet it didn't join them when the war started and it even joined the Allied side later during the war!

2: one month after Italy surrendered to Allied forces, it declared war on Nazi Germany, its onetime Axis powers partner. Italy was led into the war by Benito Mussolini, the fascist prime minister who had formed an alliance with Nazi Germany in 1936!