Simplify the following text for a Grade 12 native English-speaking person.
Original: The so-called Partition Sejm , with Russian military forces threatening the opposition , on September 18 , 1773 , signed the treaty of cession , renouncing all claims of the Commonwealth to the occupied territories .
Simplified: On September 18 , 1773 , the Committee formally signed the treaty of cession , renouncing all claims of the Commonwealth to the occupied territories .