The British were overconfident and under-prepared. Prime Minister Salisbury and his top officials, especially Colonial Secretary Joseph Chamberlain, ignored the repeated warnings of military advisors that the Boers were well prepared, well armed, and fighting for their homes in a very difficult terrain. The Boers struck first, besieging Ladysmith, Kimberley, and Mafeking in early 1900, and winning important battles at Colenso (15 December 1899), Magersfontein and Stormberg (10 December 1899). Staggered, the British fought back, relieved the besieged cities, and prepared to invade first the Orange Free State, and then Transvaal in late 1900. The Boers refused to surrender or negotiate, and reverted to guerrilla warfare. After two years of hard fighting, Britain, using over 400,000 soldiers, systematically destroyed Boer resistance, raising worldwide complaints about brutality. The Boers fought for their homes and families, which provided them with food and hiding places. The British responded by forcefully relocating all the Boer civilians into heavily guarded concentration camps, where about 28,000 died of disease, while British military forces systematically blocked off and tracked down the highly mobile Boer combat units. The battles were small operations; most of the dead succumbed to disease. The war ended in victory for the British and the annexation of both Boer republics, which became the Transvaal Colony and the Orange River Colony.