Complex sentence: John A. Macdonald, Canada's first prime minister, originally favoured a unitary system; later, after witnessing the carnage of the American Civil War, he supported a federal system to avoid similar violent conflicts.
Split into simpler sentences: John A. Macdonald, Canada's first prime minister, originally favoured a unitary system. Later, after witnessing the carnage of the American Civil War, he supported a federal system to avoid similar violent conflicts.