Martin Luther King Jr. wrote his famous "Letter from Birmingham City Jail" in April 1963 as a response to a letter written by eight Birmingham clergymen that had been published in the Birmingham News. The clergymen critiqued King's participation in the nonviolent protest that resulted in his arrest. In this letter, King justified his participation in the protest and argued that such civil disobedience is necessary for the pursuit of justice.
The letter has become a well-known text for many reasons, including its important themes, powerful use of figurative language, and memorable quotations, such as "Injustice anywhere is a threat to justice everywhere." When writing an essay about this letter, one might explore one of these elements. Or, one might take a more historical approach and examine the impact the letter had on the civil rights movement and other social movements in the United States and elsewhere.
For example, one could write an essay examining how Martin Luther King Jr. uses figurative language to enhance his argument in this letter. A possible thesis for such an essay might read: "In his 'Letter from Birmingham City Jail,' Martin Luther King Jr. uses metaphor, allusion, and imagery to emphasize the importance of peaceful protest."