The time had now come for the great combined movement which was to sweep the main Boer army off the line of the Delagoa railway, cut its source of supplies, and follow it into that remote and mountainous Lydenburg district which had always been proclaimed as the last refuge of the burghers. Before entering upon this most difficult of all his advances Lord Roberts waited until the cavalry and mounted infantry were well mounted again. Then, when all was ready, the first step in this last stage of the regular campaign was taken by General Buller, who moved his army of Natal veterans off the railway line and advanced to a position from which he could threaten the flank and rear of Botha if he held his ground against Lord Roberts. Buller's cavalry had been reinforced by the arrival of Strathcona's Horse, a fine body of Canadian troopers, whose services had been presented to the nation by the public-spirited nobleman whose name they bore. They were distinguished by their fine physique, and by the lassoes, cowboy stirrups, and large spurs of the North-Western plains.

It was in the first week of July that Clery joined hands with the Heidelberg garrison, while Coke with the 10th Brigade cleared the right flank of the railway by an expedition as far as Amersfoort. On July 6th the Natal communications were restored, and on the 7th Buller was able to come through to Pretoria and confer with the Commander-in-Chief. A Boer force with heavy guns still hung about the line, and several small skirmishes were fought between Vlakfontein and Greylingstad in order to drive it away. By the middle of July the immediate vicinity of the railway was clear save for some small marauding parties who endeavoured to tamper with the rails and the bridges. Up to the end of the month the whole of the Natal army remained strung along the line of communications from Heidelberg to Standerton, waiting for the collection of forage and transport to enable them to march north against Botha's position.

On August 8th Buller's troops advanced to the north-east from Paardekop, pushing a weak Boer force with five guns in front of them. At the cost of twenty-five wounded, principally of the 60th Rifles, the enemy was cleared off, and the town of Amersfoort was occupied. On the 13th, moving on the same line, and meeting with very slight opposition, Buller took possession of Ermelo. His advance was having a good effect upon the district, for on the 12th the Standerton commando, which numbered 182 men, surrendered to Clery. On the 15th, still skirmishing, Buller's men were at Twyfelaar, and had taken possession of Carolina. Here and there a distant horseman riding over the olive-coloured hills showed how closely and incessantly he was watched; but, save for a little sniping upon his flanks, there was no fighting. He was coming now within touch of French's cavalry, operating from Middelburg, and on the 14th heliographic communication was established with Gordon's Brigade.

Buller's column had come nearer to its friends, but it was also nearer to the main body of Boers who were waiting in that very rugged piece of country which lies between Belfast in the west and Machadodorp in the east. From this rocky stronghold they had thrown out mobile bodies to harass the British advance from the south, and every day brought Buller into closer touch with these advance guards of the enemy. On August 21st he had moved eight miles nearer to Belfast, French operating upon his left flank. Here he found the Boers in considerable numbers, but he pushed them northward with his cavalry, mounted infantry, and artillery, losing between thirty and forty killed and wounded, the greater part from the ranks of the 18th Hussars and the Gordon Highlanders. This march brought him within fifteen miles of Belfast, which lay due north of him. At the same time Pole-Carew with the central column of Lord Roberts's force had advanced along the railway line, and on August 24th he occupied Belfast with little resistance. He found, however, that the enemy were holding the formidable ridges which lie between that place and Dalmanutha, and that they showed every sign of giving battle, presenting a firm front to Buller on the south as well as to Roberts's army on the west.

On the 23rd some successes attended their efforts to check the advance from the south. During the day Buller had advanced steadily, though under incessant fire. The evening found him only six miles to the south of Dalmanutha, the centre of the Boer position. By some misfortune, however, after dark two companies of the Liverpool Regiment found themselves isolated from their comrades and exposed to a very heavy fire. They had pushed forward too far, and were very near to being surrounded and destroyed. There were fifty-six casualties in their ranks, and thirty-two, including their wounded captain, were taken. The total losses in the day were 121.

On August 25th it was evident that important events were at hand, for on that date Lord Roberts arrived at Belfast and held a conference with Buller, French, and Pole-Carew. The general communicated his plans to his three lieutenants, and on the 26th and following days the fruits of the interview were seen in a succession of rapid manoeuvres which drove the Boers out of this, the strongest position which they had held since they left the banks of the Tugela.