Simplify the following text for a Grade 12 native English-speaking person.
Original: 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 .
Simplified: 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 .