Throughout history there has rarely been a political office higher in stature and power than that of king or emperor. In republican dictatorships these titles have often proven too tempting to resist, and often at the apex of his power, a dictator will sometimes decide to proclaim himself king, and thus turn the nation into a monarchy.
In 1804 French dictator Napoleon Bonaparte decided to consolidate his power by proclaiming himself Emperor Napoleon I. Though this imperial regime would end with his fall from power, years later Napoleon's nephew Louis-Napoléon Bonaparte would be elected President of France and proceed to declare himself Emperor, as well.
In Haiti there were three such cases: Governor-General Jean-Jacques Dessalines became Emperor Jacques I (1804-06), President Henry Christophe became King Henri I (1811-20), and President Faustin Soulouque became Emperor Faustin I (1849-59).
In 1915 Chinese President Yuan Shikai declared a restoration of the Chinese monarchy, with himself as the new Emperor. The plan was a huge failure, and he was quickly forced to step down.
President Ahmet Zogu of Albania proclaimed himself "King Zog" in 1928, creating a short lived monarchy that would be eventually deposed by Italy.
A short-lived African empire was also created in 1976 when dictator Jean-Bédel Bokassa of the Central African Republic proclaimed himself "Emperor Bokassa I" and had a lavish coronation ceremony in 1977.