McCrae was an enthusiastic Imperialist at a time when Canadian nationalism had not yet solidified. When war broke out in South Africa between the British Empire and Dutch settlers, he felt it was his duty to fight.

He deferred his studies at McGill to serve with the Canadian artillery contingent fighting alongside the British in the South African War. He resumed his studies a year later, while also working in hospitals in Montreal. After graduating, he started his own medical practice, became a popular lecturer and contributed articles to medical journals. 

Although McCrae was already in his 40s when Britain went to war with Germany in 1914, he felt the call of duty once again. Wanting to fight, he used his connections from the South African War to land a position as an artillery officer and brigade surgeon.

He was in Belgium in 1915 when the Second Battle of Ypres began. The battle was terrifying, marking the first time poison gas was used in the war. The Canadians distinguished themselves by holding the lines when the French colonial troops, who received the brunt of the first gas attack, fled in horror. The second attack was aimed at the Canadians.

McCrae was busy night and day in a dressing station in a makeshift shelter now preserved as a historic site named Essex Farm. McCrae’s friend and fellow McGill student Alexis Helmer was killed by artillery during the fighting. With no padre available for the rushed funeral, McCrae conducted the service using the Anglican Burial of the Dead from memory.