The main cause of the fall of the Khmer Empire in the 15th century was a combination of internal strife and external pressures. Political instability, driven by succession disputes and a weakening central authority, weakened the empire. Additionally, invasions by neighboring kingdoms, particularly the Thai and Vietnamese, exacerbated these issues. The empire's reliance on agriculture and irrigation also led to economic difficulties as environmental factors and warfare disrupted traditional practices. The decline ultimately resulted in the fragmentation of power and the gradual loss of control over its territories.