Borneo is the third-largest island in the world. This island covers an area of approximately 748,168 km2.
Borneo is the only island in the world shared by three countries: Indonesia, Malaysia, and Brunei Darussalam. Indonesia owns about 73% of the island, while Malaysia occupies around 26% and Brunei is 1% of the island’s total land area.
This division is not a coincidence. It comes from a long history of European empires drawing lines on maps, followed by independence movements across Asia after World War II.
Britain vs. The Netherlands
This split actually started in the 1800s. At the time, Great Britain and the Netherlands competed for control of Southeast Asia.
To prevent a direct war, these two European powers signed the Anglo-Dutch Treaty in 1824 (also known as Treaty of London). This agreement divided the region into two zones. The British took the control over the Malay Peninsula and Singapore, while the Dutch focused their dominance over the Indonesian archipelago
On Borneo, this decision split the land in two. The Dutch took over the southern and eastern sections, which Indonesians now call Kalimantan. Meanwhile, the British expanded along the northern coast.
The British established protectorates along the northern coast in Sarawak and North Borneo, which is now named Sabah. In Sarawak, a British family known as the "White Rajahs" ruled the territory.
Initially, both empires only controlled the coastal areas. However, as their influence expanded deeper into the interior of the island, a formal boundary became necessary to avoid territory disputes. To solve this, British and Dutch officials signed the Boundary Convention on June 20, 1891.
This agreement was a major milestone for the region. Mapping a border through Borneo's dense, uncharted rainforests was incredibly difficult, so the surveyors decided to use natural landmarks.
They drew the line along major mountain ridges that divided the island's river systems. On the east coast, they used a specific straight line across Sebatik Island, and used local rivers to connect the missing gaps in the jungle.
While British and Dutch officials signed later updates in 1915 and 1928 to fix small errors, the 1891 line remained the main foundation. Today, this old colonial line is the exact international border separating Indonesia's Kalimantan from the Malaysian states of Sabah and Sarawak.
The Birth of Modern Kalimantan, Sarawak, and Sabah
Everything changed after World War II when European colonial rule ended in Asia. It paved the way for new independent countries, including those on Borneo.
In 1945, Indonesia declared independence from the Netherlands. Since the Dutch had ruled southern Borneo, that large territory became part of the new Indonesian republic as Kalimantan.
Moreover, in 1963, the British prepared to leave their colonies on the island. The territories on the northern coast decided to merge with the independent Federation of Malaya to form a new country called Malaysia. Today, Sarawak and Sabah make up this Malaysian side, covering around 26% of Borneo.
Why Brunei Stood Alone
There was one major exception to the Malaysian merger. Centuries ago, the Sultanate of Brunei was a powerful empire that ruled most of the Borneo coast.
However, by 1963, colonial expansion and local conflicts had significantly shrunk its territory. While Malaysia invited Brunei to join the new federation, the Sultan ultimately declined the offer.
Brunei's decision to stay out came down to two major factors: oil and power. The territory had recently discovered massive oil reserves, and joining Malaysia meant sharing this massive wealth with the federal government in Kuala Lumpur.
On top of that, the Sultan wanted to maintain absolute monarchical rule rather than transferring his authority to a distant federal parliament. Because of this, Brunei chose to remain a British protectorate for a few more decades.
The country then officially became a fully independent state in 1984. By doing so, Brunei managed to keep both its immense wealth and its traditional system of monarchy intact.
As a result, Borneo is now an island where three different nations exist side by side. Decisions made by European officials two centuries ago continue to shape the modern reality of the region.
