In December 1820 , Viceroy Juan Ruiz de Apodaca sent a force led by a royalist criollo officer , Colonel AgustÃ n de Iturbide , to fight in Oaxaca . In what was supposed to be the final government campaign against the insurgents , in December 1820 , Viceroy Juan Ruiz de Apodaca sent a force led by a royalist criollo officer , Colonel AgustÃ n de Iturbide , to defeat Guerrero 's army in Oaxaca .