Ten years after joining an uprising in Yemen against autocratic rule and an economy in shambles, the same activists find themselves on opposite sides of a war that has pushed the country to the brink of famine with dim prospects for peace.

Ahmed Abdo Hezam, 35, a fighter with government forces known by his nom de guerre Ahmed Abu Al-Nasr, was a university graduate in the agro-industrial city of Taiz when he first joined youth-led protests that ended Ali Abdullah Saleh’s 33-year rule...