In 2450, Humanity is scattered over an assortment of worlds. Some were peacefully colonized, others seized from the alien races that held them. There is no central government, only a shifting set of alliances. One thing that is shared by the worlds are the Rules of Engagement, which serve to limit combat to prevent complete destruction of colonies. Part of these rules include proscriptions against nuclear weapons, along with combative AI’s. The second point is a bit gray, as support systems such as the MUL-FS550 Fire Support Robot are close to the line. The fact that a human operator is required to pull the trigger satisfies the letter of the Rules, if not the spirit.
The various planets fight among each other for various reasons - vendettas, trade, resources,etc, but these usually are low-level engagements between professional mercenary groups. Use of planetary military elements is generally considered a dangerous escalation. The Planetary forces are generally used to defend against non-human enemies, where the Rules of Engagement do not apply.