The "Letter from Birmingham Jail," written by Martin Luther King Jr., is significant for its philosophical defense of Civil Disobedience and the urgency of addressing racial injustice. In this letter, King articulates the necessity of direct action against systemic oppression, emphasizing that waiting for justice often means never receiving it. This document solidified his role as a prominent leader and offered a moral framework that inspired activists, affirming that injustice anywhere is a threat to justice everywhere.