What significant act was signed by President Truman on July 26, 1947?

The significant act signed by President Truman on July 26, 1947, is the National Security Act of 1947. This legislation was pivotal in reshaping the U.S. military and intelligence structure post-World War II. It established the National Security Council and the Central Intelligence Agency (CIA), reflecting the need for a centralized approach to national security and intelligence activities during the onset of the Cold War.

Moreover, the act also reorganized the military under a unified command structure, leading to the creation of the Department of Defense to manage the Army, Navy, and Air Force more effectively. This act laid the foundation for the U.S. defense policy and intelligence operations that we recognize today. In contrast, the other options listed pertain to different contexts and timelines, such as defense authorization in general or post-9/11 security measures, which do not relate to the transformative impact of the National Security Act of 1947.