Shallash became a captain in the Ottoman army, serving on the Libyan and Balkan fronts in 1911 and 1912, respectively. In 1916, he joined the Arab independence movement of Sharif Hussein. Three years later, he led efforts to ensure Deir ez-Zor and its environs become part of Syria; to that end, he and his Bedouin fighters expelled the British from the city without apparent sanction from Hussein's son, Emir Faisal. He was consequently dismissed from Deir ez-Zor but continued his efforts nonetheless.