December 25th 1926 when his father (Emperor Taisho (Yoshihito)) died.
He unified the warring states and created an empire now called China
The Byzantine Empire was ruled by over 100 men and women over numerous dynasties and 1,100 years, beginning with Constantine I the Great in 330 CE and ending with Constantine XI Palaiologos in 1453...
Because they look a bit like emperors because they have things on their heads like what emperors would wear.
Military campaign to defeat all enemies and then he declared himself emperor.