The Lyran Commonwealth was a fictional interstellar state in the game BattleTech supposedly formed in the year 2341 and disbanded in 3056 to join the Federated Commonwealth. The Lyran Commonwealth was formed by the merger of the Tamar Pact the Alliance of Donegal and the Federation of Skye. Originally the capital of the Lyran Commonwealth was Arcturus, but today it is Tharkad. The Lyran Commonwealth was ruled for most of its history by House Steiner.
To prevent the Free Worlds League from discontinuing supplies for the war against the Clans to the rest of the Inner Sphere, Victor Steiner-Davion had substituted a double in place of Thomas' son, Joshua Marik, since the boy had died of leukemia despite his treatment in the New Avalon Institute of Science.
When Thomas discovered the deception, he launched a war, allied with Sun-Tzu Liao and his Capellan Confederation to retake worlds he lost to Victor's father, Hanse Davion, in the Fourth Succession War. Victor controlled the half of the Federated Commonwealth formerly known as the Federated Suns, while Kathrine controlled the half fromerly known as the Lyran Commonwealth. Thomas sent a message to Kathrine reassuring her that he had no quarrel with her, and accordingly would not attack any of her worlds, though he would understand if she sided with her brother and would bear no ill-will. Instead, Kathrine decided to give Victor no support and to effectively sever her half of the realm and christen it the Lyran Alliance (not reverting completely to the original name because she claimed that, because of Victor's actions, the word commonwealth now carried a taint.)
- Stackpole, Michael A. (1995). Bred for War. Roc Books.