August 1917 brought weather that shattered all expectation. Historical records showed the month should have been more dry than wet, but that year over three inches of rain fell, more than half of it in just the first four days. The month was so overcast and windless that standing water refused to evaporate. The preliminary bombardment scheduled for August 2 was postponed until August 10 due to rain. More delays pushed the general offensive from August 4 to August 15, then August 16. Shell craters filled with water. Supply routes dissolved. Artillery observers could see nothing through low cloud and mist. The tanks allocated to support the attack bogged down behind British lines before reaching the enemy.

The French First Army and the British XIV Corps achieved substantial success on the northern flank. French forces crossed the Yperlee Canal and drove the Germans from the Drie Grachten bridgehead, eliminating the possibility of the Fifth Army being outflanked from the north. The British 20th Division and 29th Division captured Langemarck itself, with the infantry forming up on the east bank of the Steenbeek in the pre-dawn darkness, then advancing through the village to their objectives beyond. At Schreiboom, German counter-attacks gained some ground but were ultimately contained. The French captured more than 900 prisoners and fifteen guns.

On the Gheluvelt Plateau to the south, the story was different. The 56th London Division advanced quickly through Glencorse Wood, overrunning German defenders in a sunken road, but gaps opened between brigades as units veered around impassable mud. German artillery isolated the forward troops, then counter-attacks by the 34th Division drove them back to their start line. The 16th Irish Division and 36th Ulster Division, already depleted by a third after holding the line since late July, were overwhelmed. German smoke shells masked the counter-attack from British observers, and the SOS flares fired by the infantry went unseen. By 3:30 PM, the corps commander ordered the barrage brought back to the start line, abandoning any survivors still holding out beyond it.

Mist and cloud made air observation difficult throughout the battle. Only one aircraft per corps was assigned to counter-attack patrol, far too few to track German movements across the wide battlefield. The coordination between air and ground forces that had been improving throughout the war broke down completely. On the II Corps front, no warning came of the German counter-attack. On the XIX Corps front, despite what observers called ideal visibility, aircraft failed to spot German troops massing behind smoke screens. The promised air attacks on German strong points rarely materialized. Tanks that might have crushed pillboxes sat immobilized in the mud.

The Battle of Langemarck forced a fundamental reassessment. Division commanders reported that pillboxes had not been damaged by artillery fire, that there was insufficient time to study the ground before attacking, and that the wet conditions had made supply impossible. Major-General Oliver Nugent of the 36th Ulster Division recommended slower creeping barrages with more frequent pauses, infantry formations changed from skirmish lines to company columns, and more aircraft dedicated to counter-attack patrol. The rains continued through the rest of August, and Haig transferred principal authority for the offensive to the Second Army under Plumer. When the sun finally emerged in September, the lessons of August would shape the successful bite-and-hold attacks at Menin Road Ridge, Polygon Wood, and Broodseinde.