Editor's Note: James M. McPherson is the George Henry Davis '86 Professor of History Emeritus at Princeton University. He has published numerous volumes on the Civil War, including the Pulitzer Prize-winning Battle Cry of Freedom and Crossroads of Freedom. His most recent book is Embattled Rebel: Jefferson Davis as Commander in Chief. Here, James looks at the vagaries of the US Constitution, the expanding road of presidential powers from the Civil War to drone strikes, and raises the important question: At what costs must war be won?

"As your commander in chief," President Obama told soldiers at MacDill Air Force base in Florida on September 17, "I will not commit you and the rest of our armed forces to fighting another ground war in Iraq." Yet he has committed the air force to combat missions in Iraq and Syria. American drones have carried out strikes in Yemen, Somalia, Pakistan, and Afghanistan under Obama's authority.

These events raise again the perennial question of the scope and limits of the president's powers as commander in chief. The Constitution is virtually silent on this question. It states simply that "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." It does not spell out what the president can or cannot do in this capacity. In Federalist No. 69, Alexander Hamilton tried to reassure opponents of the Constitution, who feared executive tyranny, that the commander-in-chief's power "would amount to nothing more than the supreme command and direction of the military forces, as first General and Admiral" of the nation. The vagueness of this statement clarified nothing. In 1840 the future secretary of state, Abel Upshur, deplored "the loose and unguarded terms" in which the Constitution specified presidential powers, that "would enable the President, by implications and constructions either to neglect his duties or to enlarge his powers."

Upshur's apprehensions about enlargement of powers came true during the Civil War. Both commanders in chief -- Abraham Lincoln and Jefferson Davis -- asserted prerogatives and enacted policies that established precedents which presidents have drawn upon ever since. The U.S. Congress was not in session when the war began with the Confederate attack on Fort Sumter. After calling state militias into federal service to suppress the rebellion, Lincoln took a number of steps that in effect preempted Congress's constitutional power to declare war and to raise and support armies. By executive order he proclaimed a blockade of Confederate ports, increased the size of the regular army and navy, called for three-year enlistments in the volunteer forces, and directed the Treasury to provide money to the Navy Department to buy and charter warships. In defiance of Chief Justice Roger B. Taney, Lincoln also suspended the writ of habeas corpus by presidential decree.

Lincoln's actions provoked intense opposition to him as a "tyrant" who had debauched the Constitution. In reply, he invoked his authority as commander in chief to carry out his inaugural pledge to preserve, protect, and defend the Constitution by preserving the United States. "As commander in chief of the army and navy, in time of war, I suppose I have a right to take any measure which may best subdue the enemy," said Lincoln on one occasion. He insisted in 1862 that when the war had begun a year earlier, "it became necessary for me to choose whether, using only the existing means, agencies, and processes which Congress had provided, I should let the government fall at once into ruin, or whether, availing myself of the broader powers conferred by the Constitution in cases of insurrection, I would make an effort to save it with all its blessings for the present age and for posterity."

When Congress met in special session in July 1861, it passed a resolution that "approved and in all respects legalized and made valid ... all the acts, proclamations, and orders of the President of the United States respecting the army and navy ... as if they had been done under the previous express authority and direction of the Congress." This sweeping imprimatur probably did more to legitimize the vast powers of the president as commander in chief than any other act by any branch of the U. S. government.

Jefferson Davis made no such extensive claims for himself. He suspended the writ of habeas corpus only when his Congress authorized him to do so rather than acting on his own prerogative as Lincoln did. But this comparison with Lincoln is essentially a distinction without a difference. As the leading historian of civil liberties during the Civil War, Mark Neely, has expressed it: "Abraham Lincoln and Jefferson Davis acted alike as commanders in chief when it came to the rights of the civilian populace. Both showed little sincere interest in constitutional restrictions on government authority in wartime. Both were obsessed with winning the war."

Davis also defied the Confederate Congress on another front concerning his powers as commander in chief. When the war was going badly for the Confederacy in the spring of 1862, its Congress passed a bill to create the post of general in chief with full authority over all the armies of the Confederate states. Davis regarded this bill as a violation of his own constitutional duty, and vetoed it. Davis then appointed Robert E. Lee as, in effect, a general in chief charged with "the conduct of military operations ... under the direction of the President."

When Lee became commander of the Army of Northern Virginia in June 1862, the overall conduct of military operations essentially reverted to Davis. But as the Confederacy was on the verge of total defeat in February 1865, Congress once again enacted a law creating the post of general in chief with sweeping powers. This time Davis signed it and named Lee to the position, with an understanding between the two men that Lee would exercise his powers minimally, without infringing on the president's authority as commander in chief.

None of the precedents created by Lincoln and Davis during the Civil War are constitutionally binding, of course. Differing conditions of war or quasi-war in the past 150 years have generated different requirements and policies. But the rationale asserted by Lincoln and echoed by Davis seems to have remained constant: the commander in chief believes he has "a right to take any measure which may best subdue the enemy."