The delegation of authority from the President to the Secretary of Defense in the United States is a practice that has been in place since the establishment of the Department of Defense in 1947. The National Security Act of 1947, which reorganized and unified the various branches of the military under the Department of Defense, also created the position of the Secretary of Defense. 

The President delegates various tasks and decisions related to defense and military affairs to the Secretary of Defense, given their expertise in these areas. This delegation allows the President to focus on broader policy matters and other aspects of governance. 

The Secretary of Defense (SecDef) is the head of the U.S. Department of Defense and is a member of the president's Cabinet. The SecDef has several key responsibilities that they fulfill for the president:

- Advising the president on military and defense policies, programs, and budgets - The SecDef is the president's top military advisor and helps shape defense priorities, spending, and strategy. They offer guidance on military operations, acquisition programs, force structure, and more.

- Leading and managing the Department of Defense - As head of the DoD, the SecDef oversees the Army, Navy, Air Force, Marines, and Coast Guard. They establish policies and guidance for all branches of the military. 

- Interacting with Congress - The SecDef regularly testifies before Congressional committees on defense programs and spending. They advocate for the president's defense budget requests and priorities.

- Developing the national defense strategy - The SecDef spearheads the development of high-level defense strategy that aligns with the president's national security vision. This includes force planning, contingency planning, and more.

- Overseeing international defense relationships - The SecDef manages defense ties between the U.S. and allies/partners. This includes arms sales, military exercises, base agreements, and communications during crises.

Here are some examples of how different presidents have utilized their Secretaries of Defense for defense-related issues:

- President Kennedy and the Cuban Missile Crisis - Kennedy relied heavily on SecDef Robert McNamara during the tense standoff with the Soviet Union. McNamara helped analyze options and advise on the naval blockade and negotiated the removal of missiles.

- President Johnson and the Vietnam War - SecDef Robert McNamara was one of the key architects of early escalation in Vietnam. He helped Johnson navigate military dynamics on the ground. However, he later broke up with LBJ and voiced doubts about the war. 

- President Reagan and the Cold War - SecDef Caspar Weinberger oversaw a major defense buildup under Reagan, including increasing spending, missile defense systems, and modernizing capabilities to counter the Soviet threat. 

- President Obama and the War in Afghanistan - SecDef Robert Gates played a major role in the Afghanistan strategy review in 2009. He provided options to Obama and helped shape the final decision on troop increases.

- President Trump and ISIS - SecDef James Mattis devised plans that enabled the rapid defeat of the ISIS caliphate in Iraq and Syria. He coordinated closely with Trump on rules of engagement and progress.

- President Biden and Ukraine - SecDef Lloyd Austin has been deeply involved in all military assistance programs to aid Ukraine against Russia's invasion. He has coordinated closely with allies on weapons transfers.

So in summary, the SecDef is the president's right-hand advisor on military matters while also managing the massive Defense Department bureaucracy. SecDefs shape strategy and policy at the president's direction. This fundamental role has remained consistent across administrations. In times of crisis, war, or major operations, the SecDef typically plays an integral advisory and management role for the commander-in-chief.