Henry David Thoreau is known for popularizing the concept of civil disobedience in the 20th century. His essay, "Civil Disobedience," inspired activists, including Martin Luther King Jr. and Mahandas Gandhi, advocating for the refusal to comply with unjust laws. Thoreau argued that individuals have a moral duty to resist government actions that clash with their conscience, setting a philosophical foundation for modern movements against tyranny and injustice. This idea continues to influence contemporary social justice initiatives, underscoring the importance of individual rights in the face of oppression.