Short answer for each please
Italy had signed a treaty with Germany & Austria - Hungary before WW I stating that if one of the three nations were attacked, the other two would come to help them. However, when the war started in 1911, Italy did not help these countries! Instead, the Central Powers convinced Italy to join their side in 1915. The Central Powers promised them they would be given new territories if they helped them win the war.
When it came time to sign the Treaty of Versailles (to end WW I), Italy was left out of a lot of the treaty negotiations. This was because the Central Power countries - especially Britain and France - felt that Italy didn't do their "fair share" during the war. It was true that these countries had to send troops to the Italian front to prevent the country from being overrun.
Italy agreed with some parts of the treaty, but were made about not getting the territories they were promised.