From Wikipedia
Mohamed Heikal, a chronicler of Egyptian national policy decision making and confidant of Nasser, wrote in
For Egypt Not For Nasser, that he had engaged Nasser on the subject of supporting the coup in Yemen. Heikal argued that Sallal's revolution could not absorb the massive amount of Egyptian forces that would arrive in Yemen to prop up his regime, and that it would be wise to consider sending Arab nationalist volunteers from throughout the Middle East to fight alongside the republican Yemeni forces, suggesting the
Spanish Civil War as a template from which to conduct events in Yemen. Nasser refused Heikal's ideas, insisting on the need to protect the
Arab nationalist movement.