The major conflict that Albert Einstein spoke out against was World War I and, later, World War II. Although initially seen as a pacifist, Einstein later became an advocate for civil rights and spoke against militarism and nationalism, particularly against the backdrop of World War II. He famously warned about the dangers of nuclear weapons after the atomic bomb was developed. Einstein's engagement in peace discussions and his emphasis on humanitarian values highlighted his belief that science and ethics should guide humanity away from war and towards cooperation and understanding.