John McCrae’s poem “In Flanders Fields” may have inspired the Poppy Appeal, but it was an American, specifically a Georgian, who popularized it. According to Wikipedia:

In 1918, Moina Michael, who had taken leave from her professorship at the University of Georgia to be a volunteer worker for the American YMCA Overseas War Secretaries organization, was inspired by the poem and published a poem of her own called “We Shall Keep the Faith“. In tribute to McCrae’s poem, she vowed to always wear a red poppy as a symbol of remembrance for those who fought and helped in the war. At a November 1918 YMCA Overseas War Secretaries’ conference, she appeared with a silk poppy pinned to her coat and distributed 25 more to those attending. She then campaigned to have the poppy adopted as a national symbol of remembrance. At a conference in 1920, the National American Legion adopted it as their official symbol of remembrance. At this conference, Frenchwoman Anna E. Guérin was inspired to introduce the artificial poppies commonly used today. In 1921 she sent her poppy sellers to London, where the symbol was adopted by Field Marshal Douglas Haig, a founder of the Royal British Legion. It was also adopted by veterans’ groups in Canada, Australia and New Zealand. James Fox notes that all of the countries who adopted the remembrance poppy were the “victors” of World War I.

The poem:

Oh! you who sleep in Flanders Fields,
Sleep sweet – to rise anew!
We caught the torch you threw
And holding high, we keep the Faith
With All who died.

We cherish, too, the poppy red
That grows on fields where valor led;
It seems to signal to the skies
That blood of heroes never dies,
But lends a lustre to the red
Of the flower that blooms above the dead
In Flanders Fields.

And now the Torch and Poppy Red
We wear in honor of our dead.
Fear not that ye have died for naught;
We’ll teach the lesson that ye wrought
In Flanders Fields.