The president of the United States is the commander-in-chief of the United States Armed Forces as well as all federalized United States Militia. In this capacity, the president exercises supreme operational command and control over all military personnel and militia members and has plenary power to launch, direct and supervise military operations, order or authorize the deployment of troops, unilaterally launch nuclear weapons, and form military policy with the Department of Defense and Homeland Security. However, the constitutional ability to declare war is vested only in Congress.

Article II of the U.S. Constitution expressly designates the president as:

Commander in Chief of the Army and Navy of the United States, and of the Militia of the several States, when called into the actual Service of the United States

U.S. ranks have their roots in British military traditions, with the President possessing ultimate authority, but no rank, maintaining a civilian status. Before 1947, the president was the only common superior of the Army (under the secretary of war) and the Navy and Marine Corps (under the secretary of the navy). The National Security Act of 1947, and the 1949 amendments to the same act, created the Department of Defense and the services (Army, Navy, Marine Corps, and Air Force) became subject to the “authority, direction, and control” of the secretary of defense. The president and the secretary of defense collectively, form the National Command Authority. The present-day operational command of the Armed Forces is delegated from the president to the Department of Defense and is normally exercised through its secretary. The chairman of the Joint Chiefs of Staff and the combatant commands assist with operations as outlined in the presidentially-approved Unified Command Plan (UCP).

The exact degree of authority that the Constitution grants to the president as commander-in-chief has been the subject of much debate throughout American history, with Congress at various times granting the president wide authority and at others attempting to restrict that authority. There is broad consensus that the framers of the Constitution intended Congress to declare war and the president to direct the war; Alexander Hamilton said that the president, although lacking the power to declare war, would have “the direction of war when authorized or begun”, further explaining in Federalist No. 69 that “The President is to be commander-in-chief of the army and navy of the United States. … It would amount to nothing more than the supreme command and direction of the military and naval forces … while that of the British king extends to the DECLARING of war and to the RAISING and REGULATING of fleets and armies, all [of] which … would appertain to the legislature”.

Pursuant to the War Powers Resolution of 1973, Congress must authorize any troop deployments longer than 60 days, although that process relies on triggering mechanisms that have never been employed, rendering it ineffectual. Additionally, Congress provides a check to presidential military power through its control over military spending and regulation. Presidents have historically initiated the process for going to war, but critics have charged that there have been several conflicts in which presidents did not get official declarations, including Theodore Roosevelt’s military move into Panama in 1903, the Korean War, the Vietnam War, and the invasions of Grenada in 1983 and Panama in 1989.

The amount of military detail handled personally by the president in wartime has varied dramatically. George Washington, the first U.S. president, firmly established military subordination under civilian authority. In 1794, Washington used his constitutional powers to assemble 12,000 militia to quell the Whiskey Rebellion—a conflict in western Pennsylvania involving armed farmers and distillers who refused to pay excise tax on spirits. According to historian Joseph Ellis, this was the “first and only time a sitting American president led troops in the field”, although James Madison briefly took control of artillery units in the defense of Washington D.C. during the War of 1812.

President Abraham Lincoln was deeply involved in strategy development and day-to-day military operations during the American Civil War, 1861–1865; historians have given Lincoln high praise for his strategic sense and his ability to select and encourage commanders such as Ulysses S. Grant. On the other extreme, Woodrow Wilson paid very little attention to operational military details of World War I and had very little contact with the War Department or with General John J. Pershing, who had a high degree of autonomy as commander of the armies in France. As president during World War II, Franklin D. Roosevelt worked closely with his generals and admirals, and assigned Admiral William D. Leahy as “Chief of Staff to the Commander in Chief”. Harry S. Truman believed in a high amount of civilian leadership of the military, making many tactical and policy decisions based on the recommendations of his advisors—including the decision to use atomic weapons on Japan, to commit American forces in the Korean War, and to terminate Douglas MacArthur from his command. Lyndon B. Johnson kept a very tight personal control of operations during the Vietnam War, which some historians have sharply criticized.

The Iraqi invasion of Kuwait in 1990 and the ensuing Gulf War in 1991 saw George H. W. Bush assemble and lead one of the largest military coalitions of nations in modern times. Confronting a major constitutional issue of murky legislation that left the wars in Korea and Vietnam without official declarations of war, Congress quickly authorized sweeping war-making powers for Bush. The leadership of George W. Bush during the War in Afghanistan and Iraq War achieved mixed results. In the aftermath of the September 11 attacks by al-Qaeda, the subsequent War on Terror that followed, and the 2003 invasion of Iraq due to Iraq’s alleged sponsorship of terrorism and possession of weapons of mass destruction, the speed at which the Taliban and Ba’ath Party governments in both Kabul and Baghdad were toppled by an overwhelming superiority of American and allied forces defied the predictions of many military experts. However, the insufficient post-war planning and strategy by Bush and his advisors to rebuild those nations were costly.

During the 20th century, certain regional commanders came to be called commander-in-chief. As of 2011, there are nine combatant commanders; six have regional responsibilities, and three have functional responsibilities. Before 2002, the combatant commanders were referred to in daily use as commanders-in-chief (for instance “Commander in Chief, U.S. Central Command”), even though the positions were in fact already statutorily designated as “combatant commander” (CCDR). On 24 October 2002, Defense Secretary Donald H. Rumsfeld announced his decision that the use of commander-in-chief would thereafter be reserved for the president only.

As the purpose of a military is to ward off and combat foreign adversaries, U.S. troops cannot be deployed within the United States. The only exception to this principle is if the president invokes the Insurrection Act of 1807 to quell civil turmoils, rebellions, and insurrections.

The president can – with certain limitations – call into federal service all or individual units of the National Guards and naval militias of the states to either supplement regular forces, assist state governments in the case of rebellion or insurrection, or to enforce federal law when such enforcement is impracticable by normal means. Additionally, the president also maintains direct authority over the District of Columbia National Guard. As opposed to military forces, militia units can operate on U.S. soil.

In times of war or national emergency, the Congress may grant the president broader powers to manage the national economy and protect the security of the United States, but these powers were not expressly granted by the Constitution.

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