Mobilised in Belfast at the outbreak of war, the 1st Dorsets landed at Le Havre on 16 August as part of the British Expeditionary Force. In Belgium the Battalion saw action at Mons on 23rd August and then formed part of the rearguard in the long British retreat. On the move for 16 consecutive days, they marched 220 miles. On 4th September the Dorsets reached the town of Gagny south of the River Marne and the retreat ended as the Allies managed to halt the German advance. Two days later they turned to the offensive.
A month into the advance, 12th October the Battalion was holding part of the front line near Pont Fixe, a bridge over the La Bassee Canal and came under heavy German counter-attack. Their heroic fighting at Pont Fixe cost the 1st Dorsets some 150 killed, 122 wounded and over 150 missing.
After spending the winter months of 1914-15 near Wulverghem in the Ypres salient, April 1915 found them south at Hill 60. Here, on 1st May, a German attack with chlorine gas caused heavy casualties, including 150 men killed but again the unflinching courage of the Battalion ensured that the line was not breached.
On 30th July 1915 the Dorsets moved south to the Somme, which – despite trench raids and shelling – remained a relatively quiet sector until the launch of the British offensive eleven months later. On 1st July 1916 (the start of the Somme offensive) they attacked near Authuille Wood (now Haie Wood) with 69 men having died on that day and total casualties of some 501 men killed, wounded or missing over the first three days of the battle. After several days of heavy losses, the Battalion were withdrawn north until October. In November they fought again near Beaumont Hamel in the closing stages of the offensive.
In 1917, after a long period near Nieuport, the Dorsets were thrown into the final attack at Passchendaele near Ypres. After a brief rest, they returned to the Ypres sector in Houthulst Forest but, on 27th March 1918 they were rushed to Arras to help halt an overwhelming German attack which proved be the final major German offensive of the war. During the night of 20th /21st May the Dorsets were involved in a raid at Hamelincourt, described as being one of the finest raids of the war, which cost them 77 men and 7 officers killed or wounded.
After the German offensive was halted, the final Allied advances began. In the Battle of Amiens in early August 1918, the Battalion attacked at Damery, losing 321 killed, wounded or missing. In the coming months the Battalion saw action at various places between Amiens and St Quentin. In early October the Battle of the Beaurevoir Line cost them another 315 casualties. The Dorsets’ final action was the Battle of the Sambre on 4th November 1918, when they helped force a crossing of the canal at Ors, close to Neuve Chapelle, which they had passed in their retreat from Mons in August 1914.