Simplify the following text for a Grade 9 native English-speaking person.
Original: Mexico 's independence would not be effectively declared from Spain in the Declaration of Independence of the Mexican Empire until September 27 , 1821 , after a decade of war .
Simplified: Mexico 's independence from Spain would n't be declared until September 27 , 1821 , after ten years of war . This happened with the Declaration of Independence of the Mexican Empire .