The Byzantine emperor who usurped the throne in 602 and triggered war with Sassanid Persia was Phocas. He seized power after a military revolt against Emperor Maurice and ruled until Heraclius overthrew him in 610.
Phocas had been a middle-ranking officer in the army of Thrace. In 602, Maurice ordered exhausted troops campaigning near the Danube to remain in the field through winter. The soldiers mutinied, proclaimed Phocas their leader, marched toward Constantinople, and forced Maurice to flee. Phocas was crowned emperor on 23 November 602, and Maurice and members of his family were soon executed.
The Persian king Khosrow II had previously received crucial military help from Maurice in recovering the Sasanian throne. After Maurice’s murder, Khosrow used the coup as a justification for war, presenting the invasion as vengeance for his former ally. Sasanian armies attacked Byzantine territory in 603 and began a conflict that lasted until 628.
Phocas’s reign was marked by repression, rebellion, and military crisis. Persian forces captured large parts of the eastern empire, while Heraclius’s revolt eventually brought Phocas’s downfall. Heraclius seized Constantinople and Phocas was executed on 5 October 610.