The first Anglo-Boer War took place on 13 December 1880. Three years prior to this date, the British had seized the Transvaal under Shepstone.
Following this was a period of passive resistance and repeated attempts by Boer leaders to have the invasion of the Transvaal revoked by the British.
These attempts did not work. Between 8 000 and 10 000 Boers gathered at Paardekraal, near Krugersdorp on 8 December 1880 where they elected Paul Kruger, Piet Joubert and M. W. Pretorius as their leaders, and on 13 December 1880 the leaders resolved to restore the Transvaal Republic.
The events of the 13 December 1880 thus in effect started the war and ended passive resistance.
Source: South African History Online