Complex sentence: Hinnawi became chief-of-staff of the Syrian Army and a veteran nationalist, Hashem al-Atassi, who had been president in the 1930s, to become prime minister, and then president of Syria.
Split into simpler sentences: Hinnawi became chief-of-staff of the Syrian Army. A veteran nationalist, Hashem al-Atassi, who had been president in the 1930s, became prime minister, and then president of Syria.