Prime Minister Shinzo Abe has lost his cool, leaving his countrymen dumbfounded.
The Japanese leader used his Facebook page on June 12 to finger former elite diplomat Hitoshi Tanaka by name, and to accuse him of being "unfit" to talk about diplomacy.
"Is this fellow all right?" asked the daily tabloid Gendai, referring to the prime minister.
What made Mr Abe blow his top was a long interview by the Mainichi Shimbun daily with Mr Tanaka that was published on the same day.
Mr Tanaka, who became a commentator on foreign affairs after his retirement from the foreign ministry, essentially said that, judging from what he had heard at many international conferences, foreigners see Mr Abe's diplomatic posture as indications of Japan swinging dramatically to the right.
"Japan runs the risk of being labelled a country that is self-centred and moved by intolerant narrow-minded nationalism," said Mr Tanaka.
In the interview, Mr Tanaka noted that Mr Abe had said in parliament that aggression remains undefined. Mr Abe had also hinted that he did not entirely endorse the landmark war-related declarations by previous administrations apologising for Japan's past aggression.
To top it all, Mr Abe had seen it fit to send a deputy prime minister and several ministers, not counting 160 lawmakers, to the controversial war-linked Yasukuni Shrine in April.
Such remarks and actions, said Mr Tanaka, give China and South Korea the excuse to attack Japan.
Mr Tanaka's comments clearly touched a raw nerve.
In his Facebook post, Mr Abe said the interview transported him back to 2002 when he was an aide in the then Koizumi administration and Mr Tanaka, who was then a diplomat, was acting as a conduit for negotiations between Tokyo and Pyongyang to normalise ties.
The two men were then at odds with each other.
Mr Tanaka had advocated a conciliatory line, while Mr Abe believed Japan should act tough with Pyongyang.
More specifically, Mr Tanaka had apparently advised Prime Minister Junichiro Koizumi that Japan should keep its promise to North Korea and repatriate five Japanese abducted by North Korea in the 1970s and 1980s that Pyongyang had allowed to return temporarily to Japan in October 2002.
However, Mr Abe had insisted that the abductees must not be allowed to go back to Pyongyang, a view that Mr Koizumi eventually concurred with.
Mr Abe saw the Tanaka interview as a golden opportunity to denounce his ex-rival.
"As a diplomat, he had made a crucial mistake. He is unfit to talk about diplomacy," wrote Mr Abe.
Mr Abe's post attracted over 1,000 comments from supporters, some of them even calling Mr Tanaka a "traitor" and "public enemy".
Into the fray stepped Mr Goshi Hosono, secretary general of the opposition Democratic Party of Japan, who no doubt spoke for many Japanese when he posted a comment on his own Facebook page, saying Mr Abe should not pick on an individual.
"I fear that if the most powerful person in the country criticises an individual, it would stifle freedom of speech," said Mr Hosono.
On the weekend, the Prime Minister had already flown to Europe, busily visiting a few countries prior to attending the Group of Eight (G8) summit in Lough Ern in Northern Ireland.
The tight diplomatic schedule, however, did not take Mr Abe's mind off the ongoing Facebook battle.
Using a spare moment just before going for talks with Polish Prime Minister Donald Tusk, Mr Abe penned a long and biting post against Mr Hosono, accusing the politician's criticism as being "off the mark".
"Mr Tanaka is not just another individual…Does Mr Hosono think there is nothing wrong with Tanaka's decisions and actions as a diplomat?" asked Mr Abe.
He then went on to accuse Mr Hosono of being a bad politician who does not reflect on his own actions, and also launched into an attack on past statements made by Mr Hosono, statements that, according to Mr Hosono, had been wrongly quoted.
As for Mr Tanaka, he has indicated he will not be intimidated by the prime minister. He told a television programme: "Japan is a democracy and does not clamp down on free speech. I believe that is not what Mr Abe has in mind either... I intend to continue speaking out as a private individual."
Meanwhile, Mr Hosono has promised to pursue the matter in parliament.
At least one government lawmaker is not on Mr Abe's side.
Mr Shinjiro Koizumi, the former PM's second son, told reporters: "One should not get into an argument with or criticise an individual by name (on Facebook)".
"No matter what a prime minister does, he will be criticised. He should just let it pass or sometimes take note. He should focus on producing results."
The Facebook exchange was keenly watched on the sidelines by many Tweeters, some of whom expressed disappointment with Mr Abe.
Said one Tweeter going by the username izhharanosuke: "It's a kid's row. To have such a person as the prime minister of a country and the leader of a political party is unbelievable. It just shows how low the standard of Japanese politicians is."
Incidentally, it is not the first time that Mr Abe has used his Facebook account to seek "revenge".
After Cabinet ministers visited the Yasukuni Shrine in April, a DPJ lawmaker told Mr Abe in parliament that the visit was hurting ties with China and South Korea, that it was not helping the abductee issue either and that families of abductees were despondent.
An enraged Mr Abe challenged the lawmaker to name her sources, but in vain. Not satisfied, he later wrote on his Facebook page: "The DPJ tells lies just like exhaling."
