Mercury

Mercury, the closest planet to the Sun, is a small, barren, crater-covered planet similar to Earth's Moon.

Overview
Mercury was the only planet that officially belonged to the Inner Planets after Venus was terraformed by Terrans. Its proximity to the Sun made it an uninhabitable scorched world so the Homeworlds never really cared about it. When the Neosapien War started, the Neosapiens took over Mercury without any effort but paid much more attention to it than the Terrans had, stationing a permanent garrison under Commander Glycon there. Thrax, who later became a famous Neosapien pilot, also served in these Mercury Forces. These measures turned out to be reasonable but insufficient, when two years later, the Exofleet retook Mercury and used it as a base to begin the reconquest of the Homeworlds.