Complex sentence: Even with the introduction of the Redhorn, the Zenebas forces were still at a disadvantage, and had to wait until the development of the Iron Kong before the Empire have a weapon capable of reliably defeating it.
Split into simpler sentences: Even with the introduction of the Redhorn, the Zenebas forces were still at a disadvantage. Only with the introduction of the Iron Kong did the Empire have a weapon capable of reliably defeating it.