The Lyons Thumb was a region within the Chaos March. It existed between 3057 until 3062.
The Marik-Liao invasion of 3057 and the events surrounding it led to the sundering of the Federated Commonwealth and the creation of the Chaos March. Within the Chaos March were three distinct areas; the territories that were disputed by the rump Commonwealth and the Capellan Confederation, newly-independent states, and the Lyons Thumb. The Thumb was a group of worlds in the Chaos March coreward of Terra claimed by Archon Katrina Steiner-Davion on behalf of the newly formed Lyran Alliance, yet extended into the Draconis Combine.
While attempting to reinforce the region with Lyran Alliance Armed Forces units in the face of Liao-Marik aggression, Coordinator Theodore Kurita was able to persuade Precentor Martial Anastasius Focht and Primus of ComStar Sharilar Mori to install approximately nine Draconis Combine Mustered Soldiery regiments under nominal ComStar control to maintain order. Both Katrina and Victor Steiner-Davion, both valid claimants to former FedCom worlds, publicly welcomed the Combine peacekeepers, though the Archon privately seethed over how foreign troops had effectively annexed the Thumb. Kurita's publicly-stated desire, though, was to maintain his realm's focus on rebuilding and the Clan threat.
The worlds of the Thumb existed in limbo until 3062, when mercenaries under the employ of the Kokuryu-kai attacked the Lyran Alliance world of Skye disguised as Combine troops and Imbros III and Yorii disguised as Lyran troops. Forces from the Isle of Skye attacked the alleged Combine unit on Ko, which coincided with the other attacks against the Combine. Faced with the perceived Lyran aggression, Coordinator Kurita formally annexed the Lyons Thumb and brought those worlds under direct Kuritan rule.
- FedCom Civil War sourcebook, p. 20 Skye, Ko, and Imbros and Yorii fighting leading to annexation of the Lyons Thumb.