Final answer:
The National Security Act of 1947 was the legislation that reformed the Foreign Service, transitioning it into a professional diplomatic corps and reshaping U.S. foreign policy making for active global engagement.
Step-by-step explanation:
The piece of legislation that was designed to change the Foreign Service into a fully professional diplomatic corps was the National Security Act of 1947. This act was a significant reform that restructured the way foreign policy was made and carried out in the United States, particularly by the president and the executive branch, with contributions from the national security adviser, the State Department, the Defense Department, the Department of Homeland Security, and intelligence agencies. The act fundamentally reshaped foreign policy making in the post-World War II era, recognizing that the US would have to shift from its longstanding policy of isolation to one of active engagement in global affairs. This transition was a response to the realization that traditional powers like Great Britain or France were no longer capable of policing the world, placing a greater responsibility on the United States to lead and partake in international diplomacy and peacekeeping efforts.