When the Reaper War began, a number of Crimson Chains slaver units operating independently or on long-range missions were caught far from their bases of operation with no communications. Almost universally these units attached themselves to local defense efforts against the Reapers, with many dying alongside their erstwhile victims. In other cases, due to their training and experience, the Crimson Chains came to lead or form an elite core among local forces, and after the war a number of these units became independent operators. Most commonly, however, isolated Crimson Chains units attached themselves to the nearest Blue Suns garrisons.
While the two organizations had a history of occasional tension, the large number of batarians amongst the Suns combined with the destruction of the Hegemony made them the logical choice for the wholly-batarian Chains. For their part, the Blue Suns were happy to incorporate trained and experienced soldiers into their command, particularly once it became public knowledge that the Crimson Chains had been led by undercover Special Intervention Unit officers. The partnership proved an effective one, and during the fighting most combined units forged bonds of loyalty and friendship, becoming known as 'Crimson Suns'.
In the aftermath of the war and amidst schisms within the Blue Suns leadership, it became clear that the Crimson Chains organization had survived. Some Crimson Suns defected from the Blue Suns to rejoin one of the factions of the Chains, while others remained behind. The leadership of the Blue Suns became suspicious of those who remained, fearing that they were spies or perhaps even plotting to take over, and as the three major factions of Crimson Chains began competing in traditionally Blue Sun markets the suspicion only increased. The Crimson Suns were subsequently transferred between high-risk areas, ostensibly because they were the best qualified but allegedly as part of hamfisted attempt to destroy them without angering sympathetic elements. In response, Crimson Suns began including small red chains on the shoulderpads of their Blue Suns hardsuits to distinguish themselves and identify one another.