Paul Kruger, the President of the South African Republic, issued an ultimatum on 9 October 1899, giving the British government 48 hours to withdraw all their troops from the borders of both the Transvaal and the Orange Free State, albeit Kruger had ordered Commandos to the Natal border in early September and Britain only had troops in garrison towns far from the border, failing which the Transvaal, allied to the Orange Free State, would declare war on the British government.
War was declared on 11 October 1899 with a Boer offensive into the British-held Natal and Cape Colony areas. The Boers had about 33,000 soldiers, and decisively outnumbered the British, who could move only 13,000 troops to the front line.
Meanwhile, to the north-west at Mafeking, on the border with Transvaal, Colonel Robert Baden-Powell had raised two regiments of local forces amounting to about 1,200 men in order to attack and create diversions if things further south went amiss. Mafeking, being a railway junction, provided good supply facilities and was the obvious place for Baden-Powell to fortify in readiness for such attacks. However, instead of being the aggressor Baden-Powell and Mafeking were forced to defend when 6,000 Boer, commanded by Piet Cronjé, attempted a determined assault on the town. But this quickly subsided into a desultory affair with the Boers prepared to starve the stronghold into submission, and so, on 13 October, began the 217-day Siege of Mafeking.
Lastly, over 360 kilometres (220 mi) to the south of Mafeking lay the diamond mining city of Kimberley, which was also subjected to a siege. Although not militarily significant, it nonetheless represented an enclave of British imperialism on the borders of the Orange Free State and was hence an important Boer objective. The Siege of Kimberley took place during the Second Boer War at Kimberley, Cape Colony (present-day South Africa), when Boer forces from the Orange Free State and the Transvaal besieged the diamond mining town. The Boers moved quickly to try to capture the area when war broke out between the British and the two Boer republics in October 1899.
Boer guns began shelling the British camp from the summit of Talana Hill at dawn on 20 October. Penn Symons immediately counter-attacked: his infantry drove the Boers from the hill, for the loss of 446 British casualties, including Penn Symons.
The Battle of Elandslaagte was a battle of the Second Boer War, and one of the few clear-cut tactical victories won by the British during the conflict. However, the British force retreated afterward, throwing away their advantage.
As Boers surrounded Ladysmith and opened fire on the town with siege guns, White ordered a major sortie against their artillery positions. The result was a disaster, with 140 men killed and over 1,000 captured. The Siege of Ladysmith began, and was to last several months. The Siege of Ladysmith was a protracted engagement in the Second Boer War, taking place between 2 November 1899 and 28 February 1900 at Ladysmith, Natal.
The Battle of Belmont was an engagement of the Second Boer War on 23 November 1899, where the British under Lord Methuen assaulted a Boer position on Belmont kopje. Methuen's three brigades were on their way to raise the Boer siege of Kimberley. A Boer force of about 2,000 men had entrenched on the range of Belmont kopje to delay their advance. Methuen sent the Guards Brigade on a night march to outflank the Boers, but due to faulty maps the Grenadier Guards found themselves in front of the Boer position instead.
The Battle of Modder River (known in Afrikaans as Slag van die Twee Riviere, which translates as "Battle of the two rivers") was an engagement in the Boer War, fought at Muddy River, on 28 November 1899. A British column under Lord Methuen, that was attempting to relieve the besieged town of Kimberley, forced Boers under General Piet Cronjé to retreat to Magersfontein, but suffered heavy casualties themselves.
On 10 December, General Gatacre tried to recapture Stormberg railway junction about 80 kilometres (50 mi) south of the Orange River. Gatacre's attack was marked by administrative and tactical blunders and the Battle of Stormberg ended in a British defeat, with 135 killed and wounded and two guns and over 600 troops captured.
Battle of Magersfontein was fought on 11 December 1899, at Magersfontein near Kimberley, South Africa, on the borders of the Cape Colony and the independent republic of the Orange Free State. British forces under Lieutenant General Lord Methuen were advancing north along the railway line from the Cape in order to relieve the Siege of Kimberley, but their path was blocked at Magersfontein by a Boer force that was entrenched in the surrounding hills. The British had already fought a series of battles with the Boers, most recently at Modder River, where the advance was temporarily halted. Lord Methuen failed to perform adequate reconnaissance in preparation for the impending battle, and was unaware that Boer Vecht-generaal (Combat General) De la Rey had entrenched his forces at the foot of the hills rather than the forward slopes as was the accepted practice. This allowed the Boers to survive the initial British artillery bombardment; when the British troops failed to deploy from a compact formation during their advance, the defenders were able to inflict heavy casualties. The Highland Brigade suffered the worst casualties, while on the Boer side, the Scandinavian Corps was destroyed. The Boers attained a tactical victory and succeeded in holding the British in their advance on Kimberley. The battle was the second of three battles during what became known as the Black Week of the Second Boer War. Following their defeat, the British delayed at the Modder River for another two months while reinforcements were brought forward. General Lord Roberts was appointed Commander in Chief of the British forces in South Africa and moved to take personal command of this front. He subsequently lifted the Siege of Kimberley and forced Cronje to surrender at the Battle of Paardeberg.