De Wet now set himself in person to execute the task entrusted to him by Botha of getting behind the British force in the Transvaal and breaking or interrupting the line of communication in the Free State.  He had not long to wait for opportunities.  He left Frankfort with 800 men, and on June 2 placed himself in observation near Heilbron, where Colvile was awaiting a supply column from the railway at Roodeval.  The convoy was harassed from the first by mischances.  Against Colvile’s orders it was despatched with but a small escort and without guns.  When he heard that sufficient protection could not be given, he counter-ordered the convoy, but the message did not arrive until after it had started.

On the second day of the march a body of the enemy was found blocking the road at Zwavel Kranz between Heilbron and Heilbron Road Station.  It was De Wet waiting for the convoy.

The news of its plight reached Heilbron Road Station,[45] and a relieving column was sent out, which came within four miles of Zwavel Kranz.  No firing, however, was heard, and the officer in command, hastily concluding that all was well, returned to the railway without finding the convoy, which next morning surrendered, the victim of easy-going indifference and neglect.

So far De Wet had done well, but he was only beginning his work.  The railway between Bloemfontein and Vereeniging was weakly held by regiments of militia threaded like beads on a string in posts along the line.  At Roodeval supplies and stores in large quantities, urgently needed by the Army in the Transvaal, were waiting until the bridge over the Rhenoster River, which had been destroyed by the Boers retreating before Lord Roberts, could be rebuilt.  There was scarcely a post that did not beckon to De Wet to come to it.

He was within reach of the railway at three vulnerable points, and he divided the force to attack them simultaneously; himself taking command of the raid on Roodeval, which was held by casual details of departmental troops stiffened by a detachment of militia.  Thus an important link in the chain was unable to bear a comparatively slight tension.  No one was recognized as being definitely responsible for the railway north of Bloemfontein.  The charge of it had been given to an officer who, unknown to the staff, was at the time in hospital and unable to take over his command; detachments were moved promiscuously by orders which came now from Pretoria and now from Bloemfontein; and in the chaos De Wet wriggled in between Colvile and Methuen.

On June 7 Heilbron Road Station, Rhenoster River Bridge, and Roodeval were captured in succession.  At the Bridge the Derbyshire Militia fought gallantly for several hours, but were overpowered in a hopeless position, and soon afterwards Roodeval and its accumulated booty fell into the hands of De Wet,[46] who on that day severed Bloemfontein from Pretoria for a week and added nearly 500 men to the muster-roll of his prisoners of war.