"Ballad of Birmingham" is set during the Civil Rights Movement of the early 1960s.  In 1963, racial tension in Birmingham, Alabama, escalated until a bomb exploded in an African American church on Sixteenth Street.  This explosion killed four girls; after this incident, interracial groups did organize and began working to prevent future incidents.

In the poem, the mother refuses to allow her child to participate in the marches downtown because Bull Conner, chief of Birmingham police, had the riot police and dogs and hoses to contain the crowds; ironically, she feels that her child will be safer in the segregated church in her neighborhood. But, of course, such is not the case. For, it was a turbulent time in the Deep South when the Civil Rights marches were took place. 

A lesser irony is present in the act of the girl's preparations for her attendance at church:  She has combed and brushed her hair, put on white gloves and white shoes, all of which are actually preparations for her innocent death.  "that smile was the last smile/To come upon her face."