Despite the German plans to maintain the stalemate with the French and British, Albrecht, Duke of Württemberg, commander of the 4th Army planned an offensive at Ypres, site of the First Battle of Ypres in November 1914. The Second Battle of Ypres, April 1915, was intended to divert attention from offensives in the Eastern Front and disrupt Franco-British planning. After a two-day bombardment, the Germans released of of chlorine gas onto the battlefield. Though primarily a powerful irritant, it can asphyxiate in high concentrations or prolonged exposure. Being heavier than air, the gas crept across no man's land and drifted into the French trenches. The green-yellow cloud started killing some defenders and those in the rear fled in panic, creating an undefended gap in the Allied line. The Germans were unprepared for the level of their success and lacked sufficient reserves to exploit the opening. Canadian troops on the right drew back their left flank and halted the German advance. The gas attack was repeated two days later and caused a withdrawal of the Franco-British line but the opportunity had been lost.