I cannot sit idly by in Atlanta and not be concerned about what happens in Birmingham. Injustice anywhere is a threat to justice everywhere. We are caught in an inescapable network of mutuality tied in a single garment of destiny. Whatever affects one directly, affects all indirectly…In spite of my shattered dreams, I came to Birmingham with the hope that the white religious leadership of this community would see the justice of our cause and, with deep moral concern, would serve as the channel through which our just grievances could reach the power structure. I had hoped that each of you would understand. But again I have been disappointed. —Martin Luther King, Jr., "Letter from a Birmingham Jail"

What was the purpose of King's letter?