September 19th, 1915

- General Haig of the British 1st Army issues his final orders to his corps commanders for the forthcoming offensive, scheduled to be undertaken in six days time.  The main attack will be undertaken by I Corps (commanded by Lieutenant General Hubert de la Poer Gough) in the north between La Bassée Canal and the Vermelles-Hulluch road and IV Corps (commanded by Lieutenant General Sir Henry Rawlinson) in the south between the Vermelles-Hulloch road and a large slag heap known as the Double Crassier.  Their objective is to carry the two German defensive lines and secure crossings of the Haute Deule Canal, between 8000 and 9000 yards from their starting line.  To the north, the Indian Corps and the III Corps beyond would undertake diversionary attacks between the La Bassée Canal and Aubers Ridge.

Crucial to the success of the British plan is the use of chlorine gas.  By today, 5500 cylinders containing a total of 150 tons of chlorine gas have been installed along the front of 1st Army.  It was estimated that German gas masks were ineffective after thirty minutes, and thus the plan was to send the gas over the German lines for forty minutes prior to the infantry attacking immediately thereafter.  However, the use of gas is entirely dependent on the weather conditions; without a strong wind in the right direction, the released gas may not pass over the German lines, and indeed might be blown back into the British lines.  Thus one of the most important individuals at Haig's headquarters is Captain E. Gold, a meteorologist seconded from the Royal Flying Corps, and whose responsibility is to provide daily weather forecasts to allow Haig to decide whether or not the chlorine gas can be used.  If the weather does not cooperate, Haig intends to undertake a scaled-down attack on the 25th, with the main advance occurring on the 26th or 27th, depending on the day the chlorine gas can be used.

- Given that some of the Austro-Hungarian forces initially assigned to the Serbian offensive must now stay on the Eastern Front in light of the ongoing crisis of the Austro-Hungarian armies there, Falkenhayn must find German forces that he can send to the Balkan Front in their stead.  Thus despite the growing signs of a French offensive on the Western Front, III Corps receives orders today to transfer to the Balkans.  Falkenhayn is playing a delicate balancing act, attempting to leave the absolute bare minimum of force on the Western Front to allow for the invasion of Serbia to go forward and avoid the collapse of Austro-Hungarian armies in the east.