 Early on May 6 , three Royal Navy ships sailed into Quebec Harbour . Troops on these ships were immediately sent into the city and , not long after , General Guy Carleton formed them up and marched them out to the American siege camp . General John Thomas , then in command of the American forces , had already been making arrangements to retreat , but the British arrival threw his troops into a panic . He led a disorganized retreat that eventually reached Sorel on about May 18 . 
