Why Are Malaysia and the Philippines Fighting Over Sabah?

After nearly 60 years of sitting in the backburner, the Philippines’ claim over Sabah is once again alive.
In the latest development surrounding the territorial dispute between Malaysia and the Philippines, foreign affairs secretary Teodoro Locsin reminded the world community that Sabah, the northern region on the island of Borneo, belongs to Filipinos.
Top Story: The Untold Story of 'Miss Fernandez,' the School Teacher Who Killed 200 Japanese in WWII
The Philippine government is “reactivating” the North Borneo Bureau, an agency under the Department of Foreign Affairs (DFA) tasked with handling diplomatic issues concerning Sabah.
“I have decided to reactivate the North Borneo Bureau after realizing that the rest of the world has forgotten our Sabah claim, casually designating it as another country's territory when we have not forgotten,” Locsin said in a statement during the DFA’s budget hearing at the House of Representatives on September 16.
It can be recalled that Locsin called out the United States Embassy in July when it referred to Sabah as part of Malaysia. “Sabah is not in Malaysia if you want anything to do with the Philippines,” said Locsin in a tweet.
Sabah is not in Malaysia if you want to have anything to do with the Philippines. https://t.co/dWZs1gsndE
— Teddy Locsin Jr. (@teddyboylocsin) July 27, 2020
The Basis of the Philippines’ Claim Over Sabah
The Philippines’ claim over Sabah is inherited from the Sultanate of Sulu, which transferred, to the government, all its claims and rights to Sabah so that the government can pursue this on its behalf.
Map of the Sultanate of Sulu in the 18th Century

According to the heirs of the Sultanate of Sulu, the sultanate leased Sabah to a British company operating in Malaysia in 1878. At the time, Malaysia was a British colony. When Malaysia gained independence from the British in 1957, its territorial boundaries remained vague. The question of Sabah’s ownership was left hanging in the air.
Below, the alleged lease agreement signed by the Sultan of Sulu and the British entrepreneur Baron de Overbeck, renting Northern Borneo to his company. Malaysia views this document as a cession agreement.
The Agreement Signed by the Sultan of Sulu and the British Entrepreneur Baron de Overbeck
Why Malaysia is Claiming Sabah
When the British granted Malaysia its independence in 1957, it left a crucial element missing: Malaysia territory was vague. Ownership over Sabah remained a contested issue between Malaysia, Indonesia, and the Philippines.
In 1963, Malaysia provoked the anger of the Philippines and Indonesia when it unilaterally decided to include Sabah in their territory. In a plebiscite, the Malaysian government asked Sabah residents to choose where they should belong. They voted for Malaysia.
In 1968, President Marcos of the Philippines attempted to take Sabah by force, which resulted in the infamous Jabidah Massacre which involved the killing of the Muslim operatives who refused to carry out the mission.
Every year, the Malaysian government pays the heirs of the Sultanate of Sulu P77,000. The heirs consider this rent, while the Malaysian government considers it cession payment.