Complex sentence: Deadlier escalation followed in 1978 when the South African Defence Force began crossing into Angola to strike at SWAPO positions; such actions were justified as necessary to prevent would-be'freedom fighters' from infiltrating south into Ovamboland.
Split into simpler sentences: Deadlier escalation followed in 1978; the South African Defence Force began crossing into Angola to strike at SWAPO positions. Such incursions were justified as necessary to prevent would-be'freedom fighters' from infiltrating south into Ovamboland.