Complex sentence: Once he had strengthened his control over the Indo-Gangetic plains, he decided to invade Gujarat, which was one of the wealthiest regions of India, because of its fertile soil and the Indian Ocean trade.
Split into simpler sentences: Once he had strengthened his control over the Indo-Gangetic plains, he decided to invade Gujarat. Gujarat was one of the wealthiest regions of India, because of its fertile soil and the Indian Ocean trade.