Did the Eight-Nation Alliance, comprising forces from the UK, USA, Germany, France, Russia, Japan, Italy, and Austria, use poison gas shells during their attack on Tianjin in 1900? This mystery remains unsolved to this day. This article explores four key questions: Why were the deceased found leaning against walls and not fallen over? Did the British forces employ specialized poison gas artillery? Was the gas released 'green gas' or 'chlorine gas'? And where are these poison gas shells now? Through historical analysis, it appears that the Eight-Nation Alliance likely used chlorine gas shells in Tianjin, but concrete evidence still requires further investigation. This type of weapon not only changed the face of warfare but also sparked discussions in international law.