Petros became known for his actions alongside
Russian forces in the
Van and
Bitlis regions during 1915–1916, where he earned a reputation for gallantry and for his resourcefulness in several military engagements. After the Russians entered Urmia, Agha Petros was appointed as a general with a small Assyrian force under his command. He later engaged and defeated forces of Ottoman and Kurds in a series of battles. In 1917, when the
Bolsheviks had taken control of the Russian armies and abducted
Tsar Nicholas II and his family, Agha Petros was so furious that he resigned from his commission with the Russian army, refusing to take orders from revolutionaries. He immediately took control of the Assyrian army and established his headquarters in Urmia. By 1918, Agha Petros and his Assyrian forces managed to control vast territory of
Iranian Azerbaijan, west of
Lake Urmia, where they established
self-governance. His volunteers had quite a few successes over the
Ottoman forces, notably at
Suldouze where Petros’ 1,500 horsemen overcame the forces of
Kheiri Bey's (8,000 men). Petros not only led the
Assyrian forces, but also had some limited control over
Armenian troops that fought alongside them. There was disunion in the ranks, and instead of posting a force to contain the Turks, whom he had defeated previously, he moved his forces to
Sain Qaleh as encouraged by the British officials, who had promised him military help. Nevertheless, the British did not fulfill their promises. He reached Sain Kala seven days after the British detachment retired. After the invasion of
Mosul by the
Young Turks, the Assyrian army, led by General Agha Petros, fought intensively and successfully against the Ottoman army and their Kurdish allies, and pushed them out of Mosul and the whole area, leading to Britain's control of the region. The battles are described in detail by surviving letters of Petros and
British officials. Agha Petros also had some disagreements with Mar Shimun, the patriarch of the
Assyrian Church of the East, and was often mistrusted by his family. The Allied military advisors reported that he schemed against Mar Shimun, by trying to dissuade the Allies to trust the Patriarch. However, after the murder of Mar Shimun by the Iranian
Kurdish leader
Simko, Agha Petros joined forces with
Malik Khoshaba and others in driving Simko from his stronghold at
Kuhnashahir. == Later years ==