Each country has a history of the pre-feudal era, and Japan is among them, and the title of emperor is still used in this country to this day. The Japanese emperor's writing first appeared around 673 in the Asuka Net Forewarned Order, written by their Emperor Tenmu, and the first to use the title of "Emperor" was Prince Shengde, who wrote in his letter to the Sui Emperor that the Eastern Emperor honored Emperor Baixi, meaning that they called the Sui Emperor the Emperor of The Sui Dynasty Emperor The Emperor of The Sui Dynasty, thus causing the wrath of the Sui Emperor. There is a country that does not recognize the title of emperor, and the word emperor does not exist in their Chinese species.
The country is now Korea, and the people of Korea feel that the name of the leader of Japan is the Japanese king, not the emperor, they think that the emperor is our unique existence in China, and only the king of ancient China is the real emperor. The only monarch in Korean history who has ever called Japan emperor is the only one who was forced to call him emperor when he was colonized by Japan. Before Japan was called emperor, it was actually called a king or a ruler, and in ancient China, before Qin Shi Huang was called emperor, we also called the name of king, and the title of emperor and emperor was higher than that of king.
China was the first to use the title emperor, but the emperor is not available to every ruler, and there is an article in it. Those who can be called emperors must unify the Central Plains, at least a large part of them, and those who occupy only this small piece of territory will be attacked if they want to be called emperors. Like the Three Kingdoms period, Yuan Shu obeyed the words of his subordinates and declared himself king, and Cao Cao immediately destroyed him after hearing about it. There are also neighboring feudal states that cannot be called emperors, the emperor is the supreme ruler, like Vietnam, Korea and Thailand, they are only kings, and there are still kings in Thailand.
Japan's supreme leaders used the title of emperor in order to be on par with the emperor of China's powerful country at that time, which can better reflect the strength of their country. And they not only used the title of emperor, but also learned the Chinese culture of Tianzi to call himself, once in the Sui Dynasty, the queen of Japan wrote a letter to the Sui Emperor, the letter said, the sunrise of the son of heaven to the book day is nowhere to be unharmed. This undoubtedly elevated their status as monarch, which made the Sui Emperor very angry, and after this they changed this title in their history books, changing the son of heaven to emperor.
From this point of view, the Japanese are very proud, and they use the same emperor in their language as our emperor, and the Koreans know this, so the Koreans never call their monarch emperor, because that would be equivalent to recognizing their monarch as an emperor, not a king.