The Ludlow Amendment was proposed in Congress in the late 1930s as a possible change to the United States Constitution. The Constitution clearly states that only Congress can declare war. The President, as Commander in Chief, also has the power to order the military into combat situations. The Ludlow Amendment would change this by requiring a popular referendum before Congress could formally declare war, except in the event of an invasion. However, it failed to be passed.

This amendment was proposed largely due to the fallout from the First World War. Despite the fervor that sent Americans to war in 1917, by the 1930s many had grown disillusioned with the jingoistic displays of the past. They saw how the war served to enrich a few elites while abusing the many young men who risked their lives or died in the process. Furthermore, with the Great Depression in full swing by the late 1930s, many Americans wanted the government to focus on improving the domestic situation rather than risk getting entangled in foreign affairs. As the threat of war in Europe and Asia increased over the 1930s, many Americans feared that President Roosevelt and members of Congress would be too eager to go to war abroad. They wanted a check on this, and Rep. Louis Ludlow of Indiana offered one in the form of this amendment.

From the very start, this proposed constitutional change had some powerful opponents, including the president himself. In fact, Roosevelt and his allies in Congress were able to delay a vote on this amendment for several months. There were also detractors in the press—many newspapers called the Ludlow Amendment foolish and irresponsible. After a heated debate, the House eventually rejected the amendment in a 209 to 188 vote on January 10, 1938.