Complex sentence: The pro-Nasser forces were unable to decisively defeat the royalists who by then controlled a third of the country and as the war continued many Yemenis came to resent the Egyptian presence, despite Nasser's attempts to foster economic growth and offer military support for Sallal's government.
Split into simpler sentences: The pro-Nasser forces were unable to decisively defeat the royalists controlled a third of the country by that time. As the war continued many Yemenis came to resent the Egyptian military presence, despite Nasser's attempts to foster economic growth and offer support for Sallal's government.