Why did the Boer War break out in 1899?

Answer 1

Basically, Britain wanted to incorporate the Transvaal and the Orange Free State into a federation under British control. The Boers did not want to lose control. Negotiations between them failed and Transvaal and the Orange Free State declared war on Britain. The tensions at least partially, or perhaps mostly, had to do with gold mines in those areas.

Answer 2

The Boer War broke out in 1899 primarily due to tensions between the British Empire and the Boer republics in South Africa, mainly the Transvaal and the Orange Free State. The main cause of the conflict was the desire for control over valuable gold and diamond reserves in the region, coupled with British imperial ambitions and the Boer desire to maintain their independence. The discovery of gold in the Transvaal increased tensions and ultimately led to armed conflict.