The Selenian Union is a primarily Lunar union of states in a military alliance. It is similar to the PDA and ISCC in terms of being an inter-system military organization, but take up more of an administrative and governmental roll than the other two groups.
The Selenian Union has its roots in the original Lunar colonies, primarily alliances between city states in Mare Serenitatis and Mare Insularum. The union itself was formed in 2119, though stayed small until gaining large amounts of new members after the Fourth Lunar War (2167-2174). Its first non-lunar members were the asteroids 2101 Adonis and 3753 Cruithne, who joined in the late 2170s. Several other Aten and Apollo asteroids joined the union in the following decades, notably 433 Eros in 2191 (433 Eros would infamously defect to fight for Mars during the Mars-ISCC split).