(OOC: Since we pulled a time skip, let's assume that Operation Copper King was partially successful, and that several of the rebel leaders were assassinated, including the emperor (the main target of the operation). In desperation and thirsty for revenge, the Monarchists launched an attack straight at the heart of the Republic and, with most of the Republics forces invested in the borders, tore the heart out.
With the loss of their monarch and with massive foreign armies on the borders, the rebel leaders decided it would be best to surrender to the Allies and hope they would appreciate the service they'd done them by taking out the Republic from within.)