He enrolled into the Ottoman Military Academy of Istanbul and graduated in 1902. During his time in the academy, he adopted the "nisba" (surname) "al-Hashimi" in reference to the religiously prestigious Banu Hashim tribe of Muhammad, from which his family claimed descent. He performed well at the academy and was recognized for his military skills. In 1913 he was stationed in Mosul and joined al-Ahd, an underground Arab nationalist society composed of Ottoman Arab officers. The branch in Mosul had been founded by al-Hashimi's brother Taha and was led by al-Hashimi. Among the officers under al-Hashimi's command were future Iraqi statesmen and military figures, including Mawlad Mukhlis, Ali Jawdat al-Aiyubi and Jamil al-Midfai. Through al-Hashimi as a liaison, al-Ahd developed relations with al-Fatat, an underground Arab nationalist society based in Damascus, which al-Hashimi joined in 1915 during a trip to Syria following the outbreak of World War I. Al-Fatat was allied with Emir Faisal, a leader of the Sharifian Army and a contender for establishing a monarchy over a united state consisting of the Ottomans' Arab territories.