What happened December 211 AD

The current stability of their joint government was only through the mediation and leadership of their mother, Julia Domna, accompanied by other senior courtiers and generals in the military. The historian Herodian asserted that the brothers decided to split the empire into two halves, but with the strong opposition of their mother, the idea was rejected, when, by the end of 211, the situation had become unbearable. Caracalla tried unsuccessfully to murder Geta during the festival of Saturnalia (17 December).

Finally, the next week, Caracalla had his mother arrange a peace meeting with his brother in his mother's apartments, thus depriving Geta of his bodyguards, and then had him murdered in her arms by centurions.