From Halopedia, the Halo wiki
The Trade Alliance is a union between colonies of the Unified Earth Government, presumably setting regulations that allow colonies to trade amongst each other. The "fine print" of the Trade Alliance prevented a colonial government from disallowing personnel of the UEG's United Nations Space Command to enter a colony's planetary "borders" and allowed them to claim executive autonomy. Article 3.75 of the Trade Alliance allows the UNSC to take control of a colony's planetary border security.
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