FACTBOX: Key facts about Japan's imperial system

(Reuters) - Japan's imperial family on Wednesday paid respects at the tomb of the country's wartime emperor, Hirohito, who died 20 years ago.

Below are key facts about Hirohito and Japan's monarchy.

* Hirohito headed Japan's empire during its relentless expansion across Asia in the early 20th century, and debate has simmered since about the extent of his responsibility for the military campaigns carried out in his name.

* Having formerly been treated as a god, he renounced his divine status after Japan's WWII defeat in 1945. Under the current Japanese constitution, drafted by U.S. occupation forces, the emperor became the "symbol of the state and the unity of the people."

* Current Emperor Akihito, born in 1933, has made efforts throughout his reign to reconcile Japan with its former colonies in Asia and to help it project an image as a peace-loving nation.

* The small size of the modern imperial household has led to a dearth of male heirs. Akihito's eldest son Crown Prince Naruhito has only one child -- a daughter, Princess Aiko. Under current law, women may not accede to the throne, so upon Naruhito's death it will pass first to his younger brother, Prince Akishino, then to Akishino's son Prince Hisahito.

* Traditionalists believe Japan's imperial institution is the world's oldest hereditary monarchy. Eighth-century chronicles say the sun goddess Amaterasu Omikami bequeathed her grandson a mirror, jewels and a sword, which he gave to the first emperor, Jimmu. The chronicles give Jimmu's reign as 660 B.C.-585 B.C., but there is doubt as to whether he ever existed.

* For most of the imperial institution's history, the emperor lacked direct political power and was primarily a symbolic and religious figure. Under the Meiji constitution, promulgated in 1889, the emperor became a constitutional monarch as well as a divine sovereign who was the focus of loyalty for his subjects.

(Reporting by Isabel Reynolds, Editing by Dean Yates)