On May 3, 1915 John McCrae wrote ‘In Flanders Fields’ poem

Inspiration for Flanders Fields

The day before he wrote his famous poem, one of McCrae's closest friends was killed in the fighting and buried in a makeshift grave with a simple wooden cross. Wild poppies were already beginning to bloom between the crosses marking the many graves. 
Unable to help his friend or any of the others who had died, John McCrae gave them a voice through his poem.
It was the second last poem he was to write.

Symbol of remembrance

In part because of the poem's popularity, the poppy was adopted as the Flower of Remembrance for the war dead of Britain,
France, the United States, Canada and other Commonwealth countries.
The symbolic poppy and John McCrae's poems are still linked and the voices of those who have died in war continue to be heard each Remembrance Day.

   In Flanders Fields

    In Flanders Fields, the poppies blow
      Between the crosses, row on row,
     That mark our place; and in the sky
     The larks, still bravely singing, fly
   Scarce heard amid the guns below.

    We are the dead. Short days ago
    We lived, felt dawn, saw sunset glow,
       Loved and were loved, and now we lie,
                              In Flanders fields.

    Take up our quarrel with the foe:
    To you from failing hands we throw
       The torch; be yours to hold it high.
       If ye break faith with us who die
    We shall not sleep, though poppies grow
                                In Flanders fields.

       John McCrae

For more information please see the link below.

https://www.veterans.gc.ca/eng/remembrance/people-and-stories/john-mccrae