The weather was also unusually severe, with snow in early April, which had less effect on German rearguards, who occupied billets and then blew them up when they retired. Allied troops in the pursuit suffered from exposure and shortages of supplies but had increased morale, better health (trench foot cases declined sharply) and adapted to open warfare. Draught animals suffered from the weather, short rations and overloading; the British artillery soon had a shortage of and several immobilised heavy artillery batteries. The length of the Western Front was reduced by , which needed German divisions to hold. The Allied spring offensive had been forestalled and the subsidiary French attack up the Oise valley negated. The main French breakthrough offensive on the Aisne (the Nivelle Offensive), forced the Germans to withdraw to the Hindenburg Line defences behind the existing front line on the Aisne. German counter-attacks became increasingly costly during the battle; after four days had been taken by the French armies and casualties were inflicted on German armies opposite the French and Belgian fronts between April and July. Most German casualties had been incurred during the Nivelle Offensive and were greater than any earlier Entente attack, against French casualties for the same period.