Complex sentence: At the end of May the squadron returned to Chonqing; it did not have enough serviceable Hawk 75s, and was supplemented with 9 old Hawk III biplanes from the 22nd Squadron.
Split into simpler sentences: At the end of May the squadron returned to Chonqing. Since they did not have enough Hawk 75s, they received 9 old Hawk III biplanes from the 22nd Squadron.