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Relations between Earth and the outer colonies deteriorated in the late 25th century. The frontier nature of the colonies brought about a dislike of CAA and UEG-alligned colonial governments. Due to limits in its capabilities, the United Nations Space Command left the policing of these colonies to UEG-sponsored local militias. However, the supply of weapons to these militias allowed for radical secessionists to arm themselves, leading to a series of rebellions across the outer colonies.
By 2500, the UNSC entered the conflict and besieged Athea, likely a place central to the Sundered Legion. At the end of the siege the Sundered Legion's leaders were arrested and put on trial in UEG-friendly courts. Rank-and-file members of the group were offered amnesty as a way of preventing further conflict.
The UNSC maintained a strong presense on Meridian following the Sundered Legion's defeat. Though the organised rebellion was crushed, however, insurrectionist activity was known to continue in a weaker state until the start of the Human-Covenant War.