In Chapter 2, Du Bois gives an overview of the Freedmen's Bureau's efforts to assist recently liberated Black Americans during the Reconstruction era, immediately following the Civil War. In an excerpt from this section included below, Du Bois utilizes both metaphor and idiom to emphasize for the reader just how ineffective these efforts were:
The agents that the Bureau could command varied all the way from unselfish philanthropists to narrow-minded busybodies and thieves; and even though it be true that the average was far better than the worst, it was the occasional fly that helped spoil the ointment.
In the above metaphor, Du Bois attempts to diagnose what went wrong with Reconstruction, stating that certain thieves operated as "flies" in the "ointment," spoiling the project of Reconstruction for all those benevolent actors who hoped to make a difference. Likening these "thieves" and "narrow-minded busybodies" to flies helps emphasize just how much of a nuisance these men were: buzzing around the heads of those well-meaning members of the Freedmen's Bureau, distracting them from their task of enacting meaningful change for the benefit of the Black community. Du Bois's use of a popular idiom in this passage simply underscores his point, rendering the obstacles faced by the Freedman's Bureau understandable to anyone who has ever had a fly spoil something.