The new Commander-in-chief, Lord Roberts and his chief of staff, Lord Kitchener landed in South Africa on 10 January 1900. Roberts decided to conquer the Boer Republics from the Cape Colony in keeping with their original strategy. He decided to use the western railway line for his advance and saw the relief of Kimberley as his prime objective. After achieving this he would then leave the railway line and make an eastward attack on Bloemfontein and then advance on Pretoria. From January 1900 he gathered a force of some 50 000 men for the coming campaign. To relieve Kimberley Roberts held the attention of General PA Cronje and CR De Wet with an infantry division, while French's Cavalry moved off in a wide arc, past Cronje's left flank. After a quick march French entered Kimberley on 15 February 1900. General Piet Cronje left Magersfontein with his convoy of wagons and retreated to Paardeberg where Roberts' force finally encircled him. On 27 February 1900, after ten days of intense fighting Cronje and some 4 000 men finally surrendered to the British.
Battle of Paardeberg
Cronje's surrender was a severe blow to the Boers and many burghers fled in despair. On 7 March General De Wet tried in vain to check the British advance on Bloemfontein at Poplar Grove. Three days later De la Rey's burghers offered courageous resistance at Abrahamskraal (Driefontein) but had to retreat because they were in danger of being outflanked. On 13 March 1900 Lord Roberts occupied Bloemfontein without meeting any resistance of note.
In Natal Buller at last realised that the key to success at Ladysmith lay in capturing Hlangwane Hill and the surrounding hills, south of the Thukela River and northeast of Colenso, where Botha's vulnerable left flank was entrenched. When Buller managed to capture these hills from 17-19 February the Boers' power of resistance crumbled and many dispirited burghers started leaving the front. Inspired by the news of Cronje's surrender the British finally managed to break through the Boer lines surrounding Ladysmith at Pietershoogte on 27 February1900. The Boers now retreated towards the Biggarsberg and on 28 February Ladysmith was relieved. The relief of Mafeking was only achieved by Colonels BT Mahon and Plumer on 17 May 1900.
On 17 March at a joint council of war at Kroonstad the Boers decided among others to abolish the cumbersome wagon laagers. In future they would employ mobile mounted commandos which heralded a new method of fighting.
Battle of Sannaspos
After De Wet had granted the Free State burghers a brief leave of absence they regrouped at the Sand River on 25 March 1900, inspired with new courage. De Wet, now Chief Commandant of the OFS, harassed the British by frequently attacking from the rear. Isolated British columns were among his favourite targets. On 31 March 1900 he dealt the British a severe blow when he defeated Brigadier-General RG Broadwood's forces at Sannaspos, 28 km east of the Free State capital. The British losses amounted to 159 men while the Boers lost 13 men. De Wet also managed to capture a convoy of 116 wagons. This victory managed to raise the Boers' morale and many burghers who had gone home after the fall of Bloemfontein again took up their weapons. He again met with success at Mostertshoek near Reddersburg, (4 April 1900). De Wet now decided to lay siege to the British garrison at Wepener. After an unsuccessful siege of sixteen days the Boers' were forced to retire when British reinforcements arrived.
On 3 May Roberts started his march to Pretoria but near the Vet River and Sand River the Boers attempted to halt the British advance - without any success. On 12 May Roberts and his army were in Kroonstad and on 28 May he had already crossed the Vaal River. The Boers could not prevent Johannesburg and the gold mines from falling into British hands two days later,although Lieutenant-General Ian Hamilton suffered heavy losses at the battle of Doornkop (29 May)