Which country's national anthem has the oldest lyrics?

"Kimigayo" (His Imperial Majesty's Reign) is the national anthem of Japan. Its lyrics are the oldest among the world's national anthems, and with a length of 11 measures and 32 characters, "Kimigayo" is also one of the world's shortest. Its lyrics are from a waka poem written by an unnamed author in the Heian period (794–1185), and the current melody was chosen in 1880, replacing an unpopular melody composed eleven years earlier. While the title "Kimigayo" is usually translated as "His Imperial Majesty's Reign", no official translation of the title or lyrics has been established in law.

From 1888 to 1945, "Kimigayo" served as the national anthem of the Empire of Japan. When the Empire was dissolved following its surrender at the end of World War II, the State of Japan succeeded it in 1945. This successor state was a parliamentary democracy, and the polity therefore changed from a system based on imperial sovereignty to one based on popular sovereignty. However, Emperor Hirohito was not dethroned and "Kimigayo" was still retained as the 'de facto' national anthem. The passage of the Act on National Flag and Anthem in 1999 recognized it as the official national and imperial anthem.