Simplify the following text for a Grade 12 native English-speaking person.
Original: After George Washington 's defeat at the Battle of Brandywine on September 11 , 1777 , the revolutionary capital of Philadelphia was defenseless , and that city prepared for British attack .
Simplified: After George Washington lost the Battle of Brandywine on September 11 , 1777 , the revolutionary capital of Philadelphia did not have defense , and got ready for the British to attack .