He was the King of Italy for little more than a month, that’s why he is called “the King of May”.
Humbert II of Savoy was the son of the King Victor Emmanuel II, the Chief of State who appointed Mussolini Prime Minister and escaped from Rome with the whole Government after authorizing the armistice with the Allied Powers during World War II.
Prince Humbert was more popular than his father. He even reportedly tried to stay in Rome during the crisis but was ordered by his father to leave “on his own or in cuffs”.
The anti-monarchical political parties knew this very well, so in their provisional constitutional agreements with the Crown, concluded after 1943, they forbade the King to abdicate.
The King initially eluded the prohibition by appointing Prince Humbert “Lieutenant General of the Kingdom”, some sort of Regent. Finally, in a last desperate attempt to win the 1946 referendum between monarchy and Republic, he abdicated and Humbert became King. For a month.