Key to Liberty — Piers Anthony

Shortly after the planet Charm was colonized 1,000 years ago, Earth finally suffered World War III. Almost everyone was destroyed and those remaining fell into a barbarian society when technology was lost. To prevent future wars, Earth's women took control and governed with a more sensible approach. Over the next thousand years, Earth rebuilt itself to an advanced technological power again and now needed more resources for its continued existence and growth. To fulfill their needs, Earth's lost colonies must be found and brought back under control. Now Earth has sent a ship with a planet-busting bomb to reclaim the colony Charm, and it appears that the only way Havoc and his planet can survive the invasion is if his children are sacrificed as hostages.