Complex sentence: As it was deemed impossible to take the city by force, Mangrai instead sent a merchant named Ai Fa as a mole to gain the confidence of its King Yi Ba; in time he became the Chief Minister and managed to undermine the King's authority.
Split into simpler sentences: As it was thought impossible to take the city by force, Mangrai sent a merchant named Ai Fa as a mole to gain the confidence of its King Yi Ba. In time, Ai Fa became the Chief Minister and managed to undermine the King's authority.