The Crescent Moon War was a rebellion against the Holy Kingdom of Faerghus by the nobility of the eastern region of Leicester that occurred from Imperial Year 881 to 901.

The conflict had its roots in an earlier rebellion: in Imperial Year 801, when Leicester was still part of the Adrestian Empire, its nobles similarly rebelled. The Empire found itself unable to suppress the rebellion, and the burgeoning Holy Kingdom of Faerghus seized the opportunity to increase its standing by occupying and annexing Leicester. The Kingdom's rule remained firm until 861, when King Klaus I died. His three sons became archdukes and divided the Kingdom into three territories that each would rule separately: the Eastern Kingdom, the Western Kingdom, and Leicester. Following the death of the archduke of Leicester in 881, the nobles of Leicester rejected his heir, choosing instead to try and form an alliance. Leicester declared its independence from the Kingdom on the 8th of the Horsebow Moon, ushering in the Crescent Moon War.

The Crescent Moon War wracked Faerghus and Leicester for years. Finally, in 901, both sides eliminated the remaining pockets of opposition within their respective borders and mutually disengaged. The Leicester Alliance was formally founded as a republic of nobles. Duke Riegan, who had played a pivotal role in securing Leicester's independence and whose house had previously ruled many of the Leicester nobles as an offshoot of the Blaiddyd royal family, was granted the responsibility of leading the Five Great Lords of the Leicester Alliance Roundtable from then on.

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