Outbreak of World War

At the outbreak of the First World War in August 1914, Mercer was amongst those immediately called upon by Hughes to establish and build a Canadian Expeditionary Force. Ordered to take his regiment to Camp Valcartier near Quebec City at 12.30 on 4 August, Mercer worked in his law office until 12.00 before arriving on time at the barracks in uniform.^[2] Within days he had been promoted to Temporary Brigadier-General in command of the 1st Canadian Infantry Brigade which consisted of the first four battalions of the Expeditionary Force recruited in Ontario.^[2][6] Amongst these men his militiamen were dispersed, intended to spread experience and morale amongst the raw recruits. At the end of September, after just under two months training, the brigade departed on the 20-day sea voyage to Britain as the first installment of the Canadian Expeditionary Force.^[5]

On arrival at Plymouth a British officer, Lieutenant General Edwin Alderson, took overall command of the "Canadian Contingent" as the Expeditionary Force was then known and Mercer was despatched to Camp Bustard on Salisbury Plain, where he oversaw the training and organising of the Canadian Force to ready it for fighting in Belgium and Northern France. Mercer performed well at this task and an inspection of his camp on 4 November by King George V, Queen Mary, Lord Roberts and Lord Kitchener drew the compliment "No finer physique in the British Army. A fine brigade. Splendid."^[5] Mercer's fellow brigadiers in the Expeditionary force, Richard Turner, VC and Arthur Currie commanded similar training camps and together the officers prepared their men for departure to France on 9 February 1915.

Second battle of Ypres

Dispatched for the Western Front, the Canadian Contingent was initially deployed near the Belgian town of Ypres where on 22 April Mercer's men became embroiled in the Second Battle of Ypres. A German attack on French lines had caused massive casualties through the use of poison gas and Mercer's brigade was fed piecemeal into the battle as reinforcements by poorly coordinated staff officers. In a confused and bloody encounter the untested Canadian forces held back the Germans despite being forced to wrap urine-soaked cloths around their faces to counteract the chlorine gas.^[5] The next day, Mercer's men were directed to attack an escarpment named Mauser Ridge, an operation which failed because French troops ordered to support the Canadian line did not arrive. Mercer himself travelled to the front line to witness the battle and came under fire for the first time before retiring in order to remonstrate with the French officers intended to support him.^[5] The Canadian Contingent took very heavy casualties in the battle and subsequently all three Canadian brigadiers were made Companions of the Order of the Bath for their courage and generalship in the action.^[7]

During the year the Canadian Contingent was involved in further fighting, assaulting German lines in the unsuccessful battles of Festubert and Givenchy. In the aftermath of these engagements, the Canadian Contingent was reorganised into the Canadian Corps, consisting of two divisions led by Turner and Currie who were under the overall command of Lieutenant-General Alderson.^[2] Mercer was detached from his Brigade and placed in charge of coordinating the mass of small and independent Canadian units which were steadily arriving from Canada.^[5] During the autumn of 1915 Mercer was able to shape this force (which included dismounted cavalry regiments and British Indian artillery) into an effective infantry formation and in January 1916 he was confirmed as Major-General in command of the 3rd Canadian Division, as this disparate force became.^[2][8]

Death at Mount Sorrel

In early June 1916, the Canadian Corps was again posted to trenches around the Belgian town of Ypres. The new commander of the Canadian Corps, Julian Byng, was inspecting the Corps positions in front of a German-held rise named Mount Sorrel and noted that the Canadian troops were overlooked by German positions and under constant danger of enemy fire. The Germans also seemed to be digging new sap trenches which implied that an assault was intended.^[9] Byng ordered Mercer to make a reconnaissance of the front line and draw up a plan to overrun the more dangerous German positions in a local attack. Mercer complied on 2 June, conducting a standard inspection of front line Canadian trenches at 08:30.^[5]

Shortly after Mercer had arrived in the trenches, a massive German artillery bombardment began, heavy calibre shells destroying trenches and caving in dugouts, killing many Canadian soldiers. Mercer was trapped in a dugout and then stunned by a huge shell burst which wounded most of his staff and the officers of the battalion he was inspecting. The more badly wounded among the officers were transported to an underground field hospital but Mercer was left behind with wounded aide Captain Lynam Gooderham, and so was not present when the hospital was buried by the explosion of four mines which preceded a large-scale German attack.^[5] In the confused situation, Mercer and Gooderham attempted to escape the advancing enemy but inadvertently ran into crossfire, where Mercer's leg was broken by a bullet.^[9]

Grave at Lijssenthoek Military Cemetery

Mercer and Gooderham sheltered from the continued artillery bombardment in a battered trench, both suffering from the effects of their wounds, until 01:00 on 3 June. By this stage of the battle an artillery duel had begun between the armies who were still mixed together, Mercer and Gooderham trapped in between.^[9] At approximately 02:00 a shrapnel shell exploded in the trench in which they were hiding, further wounding Gooderham and killing Mercer instantly with shrapnel wounds to the heart. Gooderham stayed with his commanding officer's body until captured by advancing German troops who buried the general's corpse in a shallow grave at Gooderham's request.^[9]

The buried field hospital was also captured by the advancing Germans and as a result nobody who knew of the general's whereabouts on the day returned to Allied lines. A staff officer later claimed that the shell which killed Mercer was British in origin, although it is unclear how this conclusion was reached.^[5] Believing Mercer to have been captured, General Currie assumed command of the battle and with the use of saturation bombardment was able to retake the lost trenches and drive the Germans back to Mount Sorrel despite heavy casualties.^[5]

On 21 June a Canadian burial party found 30 bodies in a sector of trench, amongst them General Mercer's remains, only recognisable by his uniform decoration. Mercer was buried in a military grave at Lijssenthoek Military Cemetery, later surmounted with a Commonwealth War Grave headstone.^[10]

Mercer's funeral was attended by many men of his Division and from his old regiments as well as numerous Canadian and British officers who had worked alongside him. Mercer was posthumously mentioned in dispatches for his courage under fire,^[11] the third time he had been so mentioned, and his division was taken over by Major-General Louis Lipsett who was himself killed in action two years later.^[12] Mercer is remembered as an efficient and capable organiser who never got the opportunity to demonstrate the tactical nous he had shown in training and exercises. He also remains the highest ranking Canadian officer to ever be killed in combat^[2] and reportedly by friendly fire.^[5]