Complex sentence: Piet Joubert, the Commandant-General of the Transvaal, had been incapacitated after falling from his horse, Louis Botha then assumed command of the Boers on this front.
Split into simpler sentences: Piet Joubert, the Commandant-General of the Transvaal, had been incapacitated after falling from his horse. As a result Louis Botha then assumed command of the Boers on this front.