Sulu archipelago in the context of "Mindanao"

Brunei, officially Brunei Darussalam, formally known as State of Brunei, is a country in Southeast Asia, situated on the northern coast of the island of Borneo. Apart from its coastline on the South China Sea, it is completely surrounded by the Malaysian state of Sarawak, with its territory bifurcated by the Sarawak district of Limbang. Brunei is the only sovereign state entirely on Borneo; the remainder of the island is divided between its multi-landmass neighbours of Malaysia and Indonesia. As of 2025, the country had a population of 466,330, of whom approximately 64,409 resided in the capital and largest city of Bandar Seri Begawan. Its official language is Malay, and Sunni Islam is the state religion of the country, although other religions are nominally tolerated. The government of Brunei is an absolute monarchy ruled by the Sultan, and it implements a fusion of English common law and jurisprudence inspired by Islam, including sharia.

At the Sultanate of Brunei's peak during the reign of Sultan Bolkiah (1485–1528), the state is claimed to have had control over the most of Borneo, including modern-day Sarawak and Sabah, as well as the Sulu archipelago and the islands off the northwestern tip of Borneo. There are also claims to its historical control over Seludong, in which Southeast Asian scholars believe the name of the location in question is actually in reference to Mount Selurong, in Indonesia, or Serudong River in eastern Sabah. The maritime state of Brunei was visited by the surviving crew of the Magellan Expedition in 1521, and in 1578 it fought against Spain in the Castilian War.