Complex sentence: The Eagles serve as the primary spacecraft of Moonbase Alpha, which has a fleet of them, and are often used to explore alien planets, defend Moonbase Alpha from attack, and to transport supplies and other items to and from the Moon.
Split into simpler sentences: The Eagles serve as the primary spacecraft of Moonbase Alpha, which has a fleet of them. They are primarily used to explore alien planets, defend Moonbase Alpha from attack, and to transport supplies and other items to and from the Moon.