McCrae's poem, In Flanders Fields, was among the most popular poems of World War I. He was inspired to write it on May 3, 1915, after presiding over the funeral of friend and fellow soldier Lieutenant Alexis Helmer, who died in the Second Battle of Ypres. 

In Flanders Fields was one of the most quoted poems from the war. As a result of its popularity, parts of the poem were used in efforts and appeals to recruit soldiers and raise money selling war bonds. Its references to the red poppies that grew over the graves of fallen soldiers resulted in the remembrance poppy becoming one of the world's most recognized memorial symbols for soldiers who have died in conflict. The poem and poppy are prominent Remembrance Day symbols throughout the Commonwealth of Nations, particularly in Canada, where "In Flanders Fields" is one of the nation's best-known literary works. 

A Canadian, McCrae received his M.D. from the University of Toronto in 1898. The following year he spent several months at Johns Hopkins Hospital, where his older brother Thomas was an assistant resident physician on Osler's medical service. In July 1914 McCrae sailed from Canada to France to serve as a medical officer. In 1918, while commanding the McGill Hospital at Boulogne, he died of pneumonia and meningitis.