Simplify the following text for a Grade 7 native English-speaking person.
Original: After hearing of Guan Yu 's death , Liu Bei became furious , and ordered his troops to begin preparing for war with Wu . A year later , Cao Cao died and his successor , Cao Pi , succeeded Emperor Xian to the throne , and declared himself Emperor of Cao Wei .
Simplified: After several useless battles against Liu Bei 's general Guan Yu , Cao Cao chose to ally with Sun Quan .