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>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.
-"In Fladders Fields" by John McCrae

Papaver rhoeas, the corn poppy, or Flanders has been used as a symbol of remembrance since the battles that took place in the French Flanders during WWI. It is common to see them used when honoring fallen Soldiers. In the USA the American Legion(VFW, but different) sells a bunch of little poppy ornaments around Memorial Day.

tl;dr: the war