What is the author's purpose in including the three paragraphs that begin with the line, "That summer, the summer of 1918, was blighted"?

The author's purpose in the three paragraphs your question refers to are to foreshadow coming events, describe the uncertainty of the times, and provide parallels to the events of the story.

In the first of the three paragraphs, the author gives readers insights into the lives of the narrator and his brother. Their family is dependent on the weather to produce crops and earn a living. The weather was so severe in this summer that crops were badly damaged. May and June are described as drought months, in which the crops withered and died. This creates a parallel to Doodle's weakness and withered limbs.

When Doodle and Brother follow their father out to the cotton fields, they find him with his shoulders sagging, and his chin sinking down to his chest. This is a similar posture to the one Doodle displays when he dies in the storm. The children are frightened at the sight of their father looking defeated. Their security is threatened, much like Doodle's security is threatened when his brother leaves him. When their father raises a fist and begins cursing the Republican party, the brothers giggle and go back to the house. They know their dad is no longer defeated and their security is restored. This parallels their victories in the face of defeat when Brother teaches Doodle to walk, which they reveal on his sixth birthday.

This paragraph also describes a July hurricane, which contains such power that it uproots oak trees and elm trees. This parallels the storm which brought the scarlet ibis to their home, as well as the storm in which Doodle dies.

In paragraph two, Chateau-Thierry, Amiens, and Soissons all refer to battles that took place in World War I in France. They parallel the efforts of Brother and Doodle in several ways. The Battle at Chateau-Theirry was fought with American and French forces collaborating successfully to defeat the Germans. Prior to this battle, fought around the regions of Soissons and Chateau-Thierry, allied forces were able to keep their plans a secret from the Germans. This parallels Brother and Doodle's plans for Doodle's development, which they kept hidden from their family. The battle of Amiens is famous for the gains made by allied forces. Allied forces gained seven miles in one day, which was one of the greatest advances of the war. This is the battle that led to the end of the war. The Battle of Amiens also notably affected German morale, and large numbers of Germans surrendered during this time. This also parallels Brother and Doodle's great gains and their stagnation in Doodle's development program. They both suffer a great loss of morale.

The third paragraph circles back to the opening line of the story, describing the clove of seasons. The preceding paragraphs describe what has lead up to the clove of seasons in which Doodle's death takes place. This paragraph shows Doodle and Brother's frustration. It describes how the narrator's pride manifested in cruel behavior directed toward Doodle, and it foreshadows his death from physical over-exertion.