Article II, Section 2, Clause 1:
The President shall be 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; he may require the Opinion, in writing, of the principal Officer in each of the executive Departments, upon any Subject relating to the Duties of their respective Offices, and he shall have Power to grant Reprieves and Pardons for Offences against the United States, except in Cases of Impeachment.
While the President customarily delegates supreme command of the forces in active service, there is no constitutional reason why he should do so, and he has been known to resolve personally important questions of military policy. Lincoln early in 1862 issued orders for a general advance in the hopes of stimulating McClellan to action; Wilson in 1918 settled the question of an independent American command on the Western Front; Truman in 1945 ordered that the bomb be dropped on Hiroshima and Nagasaki. 1 As against an enemy in the field, the President possesses all the powers which are accorded by international law to any supreme commander.
He may invade the hostile country, and subject it to the sovereignty and authority of the United States. 2 In the absence of attempts by Congress to limit his power, he may establish and prescribe the jurisdiction and procedure of military commissions, and of tribunals in the nature of such commissions, in territory occupied by Armed Forces of the United States, and his authority to do this sometimes survives cessation of hostilities. 3 He may employ secret agents to enter the enemy’s lines and obtain information as to its strength, resources, and movements. 4 He may, at least with the assent of Congress, authorize commercial intercourse with the enemy. 5 He may also requisition property and compel services from American citizens and friendly aliens who are situated within the theater of military operations when necessity requires, thereby incurring for the United States the obligation to render
just compensation. 6 By the same warrant, he may bring hostilities to a conclusion by arranging an armistice, stipulating conditions that may determine to a great extent the ensuing peace. 7 He may not, however, effect a permanent acquisition of territory, 8 though he may govern recently acquired territory until Congress sets up a more permanent regime. 9
The President is the ultimate tribunal for the enforcement of the rules and regulations that Congress adopts for the government of the forces, and that are enforced through courts-martial. 10 Indeed, until 1830, courts-martial were convened solely on the President’s authority as Commander in Chief. 11 Such rules and regulations are, moreover, it seems, subject in wartime to his amendment at discretion. 12 Similarly, the power of Congress to
make rules for the government and regulation of the land and naval forces (Art. I, § 8, cl. 14) did not prevent President Lincoln from promulgating, in April, 1863, a code of rules to govern the conduct in the field of the armies of the United States, which was prepared at his instance by a commission headed by Francis Lieber and which later became the basis of all similar codifications both here and abroad. 13 One important power that the President lacks is that of choosing his subordinates, whose grades and qualifications are determined by Congress and whose appointment is ordinarily made by and with the advice and consent of the Senate, though undoubtedly Congress could if it wished vest their appointment in
the President alone. 14 Also, the President’s power to dismiss an officer from the service, once unlimited, is today confined by statute in time of peace to dismissal
in pursuance of the sentence of a general court-martial or in mitigation thereof. 15 But the provision is not regarded by the Court as preventing the President from displacing an officer of the Army or Navy by appointing with the advice and consent of the Senate another person in his place. 16 The President’s power of dismissal in time of war Congress has never attempted to limit.
The Commander-in-Chief a Civilian Officer
Is the Commander-in-Chiefship a military or a civilian office in the contemplation of the Constitution? Unquestionably the latter. An opinion by a New York surrogate deals adequately, though not authoritatively, with the subject:
The President receives his compensation for his services, rendered as Chief Executive of the Nation, not for the individual parts of his duties. No part of his compensation is paid from sums appropriated for the military or naval forces; and it is equally clear under the Constitution that the President’s duties as Commander in Chief represent only a part of duties ex officio as Chief Executive [Article II, sections 2 and 3 of the Constitution] and that the latter’s office is a civil office. [Article II, section 1 of the Constitution . . . .] The President does not enlist in, and he is not inducted or drafted into, the armed forces. Nor, is he subject to court-martial or other military discipline. On the contrary, Article II, section 4 of the Constitution provides that ‘The President, [Vice President] and All Civil Officers of the United States shall be removed from Office on Impeachment for, and Conviction of Treason, Bribery or other high Crimes and Misdemeanors.’ . . . The last two War Presidents, President Wilson and President Roosevelt, both clearly recognized the civilian nature of the President’s position as Commander in Chief. President Roosevelt, in his Navy Day Campaign speech at Shibe Park, Philadelphia, on October 27, 1944, pronounced this principle as follows:–‘It was due to no accident and no oversight that the framers of our Constitution put the command of our armed forces under civilian authority. It is the duty of the Commander in Chief to appoint the Secretaries of War and Navy and the Chiefs of Staff.’ It is also to be noted that the Secretary of War, who is the regularly constituted organ of the President for the administration of the military establishment of the Nation, has been held by the Supreme Court of the United States to be merely a civilian officer, not in military service. (United States v. Burns,  17