Answer
Foreign invasion and rebellion caused the downfall of Phocas's reign. Phocas was a usurper who had murdered the legitimate emperor, Maurice, and taken his throne. Phocas was a brutal ruler who was hated by many people, including the military. In 610, a group of Byzantine soldiers led by Heraclius, the son of Maurice, rebelled against Phocas. They defeated Phocas's army and captured him.