Japan's emperor left for Vietnam on Tuesday to meet families of Japanese soldiers sent there by his father more than seven decades earlier during World War II.
The 83-year-old Akihito and his wife, Michiko, are making their first visit to the country, the latest in a series of trips to former battlegrounds.
"It is our hope that our visit to Vietnam will contribute to the further development of the mutual understanding and friendly relations between our two countries," Akihito said at Tokyo's Haneda airport.
Akihito and Michiko are scheduled to visit Hanoi and Hue, according to the Imperial Household Agency.
On Thursday, they are set to meet with family members of some of the Japanese soldiers who stayed in Vietnam after WWII.
Many of the 700 soldiers joined forces with Vietnamese revolutionary leader Ho Chi Minh in his fight for independence from colonial ruler France.
They were ordered to return home by Japanese authorities after France was defeated in 1954, but the first wave was not allowed to bring their Vietnamese wives or children.
Their families were often left in poverty and faced accusations of treachery for setting up house with the former enemy.
After Vietnam, the pair fly on to Bangkok on Sunday to pay respects to King Maha Vajiralongkorn, who came to the throne after the death of his father Bhumibol Adulyadej in October.
The tour comes after Akihito suggested last year he is ready to step down, citing his age and increasing frailty.