Joshua Marik was the designated heir to the throne of the Free Worlds League. The only legitimate heir of Thomas Marik's stand-in, he was crippled with leukemia from a young age. In desperation, the false Marik worked out a deal with Hanse Davion of the Federated Suns, where young Joshua would be treated at the New Avalon Institute of Science in exchange for 'Mech upgrade kits for the AFFC.
Sadly, Joshua Marik died from treatment of his leukemia on New Avalon in 3057. Due to the deaths of Hanse Davion and Melissa Steiner, the throne of the Federated Commonwealth was occupied by the young and then-inexperienced Victor Steiner-Davion. Victor proceeded with a plan his father had created, dubbed Operation Gemini, to replace the young Marik with a body double. This would prove to be an ill-advised decision however, as Joshua's death set in motion the sequence of events that would lead to the FedCom Civil War.
The discovery of his son's death and substitution is what prompted Thomas to ally with the Capellan Confederation and attack the Federated Commonwealth. The attack gave Katherine Steiner-Davion the excuse she needed to break off the Lyran half of the Commonwealth and create the Lyran Alliance, which years later led directly to the FedCom Civil War.