The overall strength of Earth's defenses still lies with the Moon. The comprehensive combat power of the Han Star Fleet surpasses that of the other three major war zones combined. The variety and proportion of warships in its combat groups represents the pinnacle of human technology. Their overall combat readiness is comparable to their performance in the Starship S race, where the Moon's dominance is absolute and only marred by the higher attention received by the Mecha S race.
In reality, as things stand now, the Moon's fleet is indeed humanity's main line of defense.