Following the German invasion of Poland in September 1939, the British Expeditionary Forces (BEF), under Lord Gort, assembled along the France-Belgium border to the left of the French First Army. The focus for the British was to fortify their 45-mile (72-km) stretch of this French northeastern front and protect the key industrial centres of Lille and Roubaix. The Gort Line was a series of concrete bunkers, pillboxes and anti-tank ditches built by BEF troops, often labouring in the cold. The 3rd and 4th Divisions covered the eastern approaches to Lille; the 1st and 2nd Divisions were deployed further east; the 5th Division arrived later on during 1939. The strategy of the Dyle Plan, or Plan D – that in the event of German attack the British and French forces would cross the border and move east to the River Dyle (Dijle) – meant that the defences built for the Gort Line were never used as intended.