Final answer:
The most significant developments that led to changes in presidential power during the 20th century included the passage of the War Powers Act, Supreme Court decisions on inherent presidential powers, and the increasing use of executive orders and agreements. The rise of the U.S. on the international stage and the ability of presidents to leverage mass media also expanded presidential authority substantially.
Step-by-step explanation:
The developments that most led to changes in the level of presidential power during the 20th century are multifaceted. The passage of the War Powers Act in 1973 is one such development that aimed to curb presidential power by requiring the president to notify Congress within 48 hours of sending troops into hostile areas without a declaration of war. The Supreme Court has historically played a role in expanding and limiting presidential power through its rulings on whether presidents have inherent powers, particularly in foreign policy.
Moreover, the increasing reliance on executive orders, and executive agreements in foreign policy, has allowed presidents to bypass Congress in some instances, thereby increasing the executive branch's decision-making capabilities. Additionally, as the nation grew, the federal government, and by extension the presidency, became more involved in the complex domestic economy, and the United States' international profile rose, leading to a more significant role for the president both at home and on the world stage.
These changes, aided by technological advancements like radio, television, and later social media, enabled presidents to directly communicate with the public and assert a prominent role, especially during critical events such as the Great Depression, two World Wars, the Cold War, and the war on terrorism. Congressional delegation of authority and the bureaucratic interpretation of legislation further enhanced presidential authority in legislative matters, including budget framing and policy implementation.