Complex sentence: Participating on the Allied side were Arabs who participated in the Arab Revolt, Armenian militia who participated in the Armenian Resistance, with the Armenian volunteer units and Armenian militia formed the Armenian Corps of the First Republic of Armenia in 1918.
Split into simpler sentences: Participating on the Allied side were Arabs who participated in the Arab Revolt, and Armenian militia who participated in the Armenian Resistance. The Armenian volunteer units and Armenian militia formed the Armenian Corps of the First Republic of Armenia in 1918.