Malaysia was invaded earlier this month. A ragtag group of people - some of them armed - travelled from the Philippine islands of Sulu to Malaysian Borneo to stake their claim on the province of Sabah.
This so-called Royal Army of Sulu, just a few hundred in number, is hardly likely to be a major threat to the Malaysian police, who are currently surrounding their base in a little village.
But the fate of these people and how their claim is handled - by both countries - may have important consequences for regional stability.
Sale or lease?
The leader of the group is the brother of Jamalul Kiram III, one of the two main claimants to the title of Sultan of Sulu.
It is a title that goes back to before the Philippines was an American colony, or a Spanish colony, or indeed properly recognised as the Philippines at all.
The two main sultanates in the region at the time were Sulu and Brunei. In 1658, the Sultan of Brunei gave Sabah to the Sultan of Sulu - either as a dowry or because troops from Sulu had helped him quell a rebellion.
More than 350 years later, the sultan's heirs have come to remind Malaysians that they still consider Sabah to be part of Sulu and, by extension, part of the Philippines.
"Sabah is our home," they said simply when asked why they had come.
But history is not that simple and of course Malaysia has no intention of giving up Sabah to this little band of Filipinos.
The crux of their disagreement lies in a contract made in 1878, between the Sultanate of Sulu and the British North Borneo Company.
Under this contract known as pajak, the company could occupy Sabah in perpetuity as long as it paid a regular sum of money.
Even today, Malaysia pays about 5,000 Malaysian ringgit (£1,000, $1,500) a year to the Sultanate of Sulu.
But the British and, after that an independent Malaysia, interpreted pajak to mean sale, while the Sulu Sultanate has always maintained it means lease.
"In my opinion, this is more consistent with a lease rather than a sale, because you can't have a purchase price which is not fixed and which is payable until kingdom come," said Harry Roque, a law professor at the University of the Philippines.
The issue has been a stumbling block in relations between Malaysia and the Philippines for decades, and a factor behind the continuing violence and instability on the islands of Sulu.
Successive Philippine presidents have pressed the sultanate's case, the most audacious being an attempt by the late President Ferdinand Marcos to train and equip a secret Muslim militia to take Sabah by force.
The plan was leaked before it could be put into action, and the militia force was allegedly killed by the Philippine army in an attempt to cover up the evidence. The massacre became one of the main triggers for rising Muslim discontent and the emergence of Muslim rebel groups which are still around in the region today.
Subsequent attempts to settle the issue have been far more peaceful and diplomatic in nature, and even the previous president, Gloria Arroyo, had brought up the claim with Malaysia on several occasions.
But under the current president, Benigno Aquino, the Sultanate of Sulu's ancestral rights have not been mentioned at all.
And that could well be why the Royal Army of Sulu decided now was the time to launch their brave, if somewhat, foolhardy invasion.
According to Mr Roque, Mr Aquino has not pursued Sulu's claims because he has been prioritising talks with a Muslim rebel group in the region, the Moro Islamic Liberation Front (MILF), instead.
These talks have been fruitful, and there is a framework peace deal in place for the first time in decades.
But the facilitators of the talks are the Malaysians - and Mr Roque says Malaysia is hardly a disinterested party.
"The fact that Malaysia volunteered to be a facilitator must have an impact on why the Aquino government has decided to keep the claim dormant," he said.
"Perhaps the Malaysians volunteered precisely because they don't want the Sabah claims to be revived."
But even if Mr Aquino does not want to deal with the Sabah issue right now, he knows he cannot just ignore Sulu's claims.
The heirs to the sultanate are highly respected, and could call on a lot more support than the few hundred people currently in Sabah if necessary.
"If the sultan's family are not included in peace talks, and feel like they're being forgotten and left out, there will soon be a serious problem," said Professor Benito Lim, a historian from Ateneo de Manila University in the capital.
I know myself how revered sultans are in Sulu. There are two main families which can claim the title of Sultan of Sulu - the Kirams and the Bahjins.
I visited a member of the Bahjin family, the Sultan of Patikul, Jainal Abirin Bahjin, in his little wooden house. He's a softly spoken man, very unassuming and welcoming, living a simple life by the beach.
But the locals who came to visit with us were clearly extremely honoured to be in his presence.
After we chatted for a while, he invited me to a private room at the back of his house, where he took out what looked like a pile of old clothes.
But inside several layers of material, there was a ceremonial sword - a gift from the sultan of Brunei to his forefathers more than 300 years ago.
Decades and centuries may go past, but this family remembers its history as clear as if it were yesterday.
No peace deal, no change of presidency, not even the insurmountable odds posed by the Malaysian security forces, are going to make them forget that Sabah used to belong to Sulu - and in their minds, still does.