Iturbide's assignment to the Oaxaca expedition coincides with a successful military coup in Spain against the new monarchy of Ferdinand VII.

The coup leaders, who had been assembled as an expeditionary force to suppress the American independence movements, compel a reluctant Ferdinand to sign the liberal Spanish constitution of 1812.

When news of the liberal charter reaches Mexico, Iturbide sees in it both a threat to the status quo and an opportunity for the criollos to gain control of Mexico.

Ironically, independence is finally achieved when conservative forces in the colonies choose to rise up against a temporarily liberal regime in the mother country.

After an initial clash with Guerrero's forces, Iturbide switches allegiances and invites the rebel leader to meet and discuss principles of a renewed independence struggle.