Final answer:
The largest demonstration in the country's history in 1969 was by antiwar protestors.
Step-by-step explanation:
The largest demonstration in the country's history held in Washington, D.C. in 1969 was by antiwar protestors. This demonstration, known as the Moratorium to End the War in Vietnam, brought together people from various backgrounds to protest against the Vietnam War. It was organized by a coalition of antiwar groups and attracted hundreds of thousands of participants.
Learn more about antiwar protest