The Last Flower was written as Thurber began to realize the sorrow and chaos of war, as can be read via the dedication to his only child “in the wistful hope that her world will be better than mine.”
The Houghton Mifflin Chronology of US Literature (2004) gives other details for the inspiration for the book and the eventual moral; it states that the book was “inspired by the Spanish Civil War and the Nazi and Soviet invasion of Poland. Thurber presents a parable of the folly of war in which the only survivors of World War XII are a man, a woman, and a flower. From these three love emerges, leading to family, tribe, civilization, and inevitably, another war.” 
While at the New York Algonquin Hotel, Thurber wrote and drew The Last Flower on their yellow paper. Both the writer and Helen, his wife, considered it to be the favorite of his twenty-six books. The book was an immediate success.
The New York Times called it “One of the most serious and yet one of the most hilarious contributions on war.” (citation o/s)
E. B. White wrote in a New Yorker feature article of November 11, 1961, “In it you will find Thurber’s faith in the renewal of life, his feeling for the beauty and fragility of life on earth.” He also wrote “Of all the flowers, real and figurative…the one that will remain fresh and wilt-proof is the little flower…on the last page of that lovely book.”
The novelist Ellen Glasgow, captivated by the book, wrote to Thurber, “I found that I had forgotten your wonderful birds. How is it possible to put so much expression into a single curve?” 
Thurber’s work “began where the other cartoonists left off,” claimed the German artist George Grosz. It was rumored that Henri Matisse said, “the only good artist in New York is a man named Thurber.” 
It was translated into French by Albert Camus and published by Gallimard in 1952.