The accurate sequence showing the development of poison gas in World War I is:
Chlorine gas released in cylinders at Ypres > British gas themselves by accident due to shift in wind > development of shells to release chlorine gas at long range.
This sequence reflects the historical events where chlorine gas was first effectively used by Germany in the Second Battle of Ypres in 1915, leading to the development of gas shells that could disperse gas more effectively on the battlefield. The incident of troops being affected by their own gas due to wind shifts highlights the dangers and unpredictability of chemical warfare.