1901- Aaron van Leiden smoothly exhaled, stopped, lifted the rifle imperceptibly and fired. Six hundred meters away a British soldier's head exploded. Six other commandos charged at the blockhouse, throwing white phosphorus as cover. A British soldier poked his head over the roof, not realizing that the Boers had telescopic sights. The five remaining soldiers in the blockhouse attempted to run out the door, getting cut down with Maxim fire. Within minutes the bridge was mined, and the commandos retreated.
(OTL the Boers would have been unable to do this. Let's assume they figure out how to transport light machine guns around and get better at sniping as well as more organized).