Complex sentence: After an initial sally against him in late 1878 proved ineffective, a larger force in November 1879 overran his fortress, whose defenders were killed almost to a man, largely by African soldiery.
Split into simpler sentences: After an initial sally against him in late 1878 had proved ineffective, a larger force overran his fortress in November 1879. The defenders of the fortress were killed almost to a man, largely by African soldiery.