Castilian War in the context of "Sultanate of Sulu"

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.

The history of Brunei concerns the settlements and societies located on the north coast of the island of Borneo, which has been under the influence of Indianised kingdoms and empires for much of its history. Local scholars assume that the Islamisation of Brunei started in the fifteenth century with the formation of the Bruneian Empire, a thalassocracy that covered the northern part of Borneo and Sulu. At the end of the 17th century, Brunei entered a period of decline brought on by the Brunei Civil War, piracy, and European colonial expansion. Later, there was a brief war with Spain, in which Brunei evacuated its capital for a brief period until the Spanish withdrew. The empire lost much of its territory with the arrival of the Western powers, such as the Spanish in Luzon and Visayas and the British in Labuan, Sarawak, and North Borneo. The decline of the Bruneian Empire accelerated in the nineteenth century when Brunei gave much of its territory to the White Rajahs of Sarawak, resulting in the empire's separation into two parts. Sultan Hashim Jalilul Alam Aqamaddin later appealed to the British to stop further annexation in 1888. In the same year, the British signed a "Treaty of Protection" that made Brunei a British protectorate until 1984 when it gained independence and prospered due to the discovery of oil.