The field commanders of armies are notoriously unreliable. In fact, the Romans forbade them from sending their armies across the Rubicon lest they try to overthrow the government. Troops were, until recently, more loyal to their commanders than they were to their polity, perhaps because it was the military units that enforced military discipline. You could be killed for insubordination. That was the power over you, not the politicians in Rome. What is remarkable and surprising, however, is that the autonomy of field commanders to do what they wanted with their troops lasted until recent times-- the First World War, I would say. This is partly because armies in the past relied on their own supplies and the funds provided to them to keep them in the field. But twentieth century armies had come to rely on supplies of oil and munitions and tanks supplied for them by their various defense ministries and so were no longer autonomous. How this balance of field command and high command alters over the past two hundred years explains a lot about the wars fought during that time period.
Remember that the Continental Congress kept two armies in the field, under financing both of them lest one of them decided to do a Cromwell and wrest power from the Congress. The two armies pursued very different policies. Washington was out to avoid a devastating defeat and Gates was all out on confronting Burgoyne. The decision about what how the army should be used was left to its general. The same was true on the British side. Though London expected Clinton and Burgoyne to split the colonies by connecting up with one another, Clinton never dispatched troops to go up the Hudson from New York to assist Burgoyne. What to do with an army was here also left to the general on the scene. Napoleon solved this problem by becoming his own field commander. He was the one who ordered the supplies for the troops and was there on the battle scene. This unity of command may account in part for his military successes.
And so it remained until the American Civil War, when things began to change. We are aware to the extent to which appointments to the rank of general was a concession to a local political leader, and given the great expansion of the Union Army, there were plenty of such commissions to go around. But more important was the fact that generals, once assigned to a sphere of control by being given command of the Union Army in that area, were free to plan their operations as they saw fit. McClellan was urged by Lincoln to attack but, apparently, Lincoln could not make him do so. Grant took it into his own head to go after Fort Henry and Fort Donelson and made separate arrangements with Admiral Foote so that he could carry out his audacious plan. The only thing Washington could do to control its generals was to reassign them to a different command or to take away control of the army they controlled. This is precisely what General Halleck, then serving as Chief of the Armies, did with Grant, for reasons having to do with personal pique, right after Grant’s victories, before restoring him to duty and so allowing him to put together his Shiloh campaign. But these formal matters of appointment and the reorganization of military districts had to do the work of coordinating the activities of the Union generals.
Things had really changed by the time of the First World War. Not only was the German General Staff in charge of war planning; it also directed how those plans unfolded in the field, and so were the ones to determine that the German Army would not take Paris according to its timetable and so should engage in a race to the Channel with the French and so set up the conditions for the trench warfare that would define the Western Front for the rest of the war. The French did control their own armies but were always in negotiations with their allies about what part of the front each would cover, Pershing, the American commanding general, insisting, when he came on the scene, that his troops would not be absorbed into French or British units to fill in for lost manpower but would pursue their own independent operations on the territory to which they were assigned.
Incredibly, the autonomy of field commanders remained a problem throughout the Second World War, even though Allied strategy was formulated by Washington and London. Eisenhower is properly credited with coordinating a multi-national alliance despite having some fractious field commanders, like Montgomery, who at one point was required to apologize to Eisenhower for overstepping his bounds because Montgomery was aware that if Eisenhower took up their disagreement with London, Churchill would have picked Eisenhower, Churchill having helped decide that Eisenhower would lead the invasion of Europe partly because he thought he could manipulate him but had come to see him as a worthy and honorable man. Eisenhower had to discipline George Patton but was not about to relieve him because he needed his skills as a field commander, whatever his deficiencies as a strategic mind. And Eisenhower could control Patton because it was SHAEF headquarters that decided how much oil would be provided to Patton’s tanks. Hitler had done the same thing, prevented Rommel from pursuing a very different grand strategy than an advance through Russia, by withholding supplies so that Rommel could not advance east and so open the way to Iranian oil.
Things were more difficult in the Pacific. Roosevelt flew out to Pearl Harbor in 1944 to meet with MacArthur, one of his field commanders (rather than order his nominal subordinate to meet him in Washington). MacArthur was insisting that he be allowed to invade the Philippines even though Pentagon planners thought going the northern route, under Admiral Halsey, was the more advisable plan. MacArthur got his way. He could not be cajoled by Roosevelt’s charm and Roosevelt, ever aware of public opinion, knew that MacArthur was very popular at home. It took Truman, eight years later, to draw MacArthur's horns by simply firing him with the assertion the the President and not a field commander is the Commander in Chief, something that Lincoln, himself no mean evaluator of public opinion, had only gingerly approached in the case of McClellan.
The Cold War saw the clear victory of Washington over its field commanders even if there were many nightmare scenarios that developed about what would happen if a field commander went rogue. It wasn’t just Dr. Strangelove fantasies, though. Multiple redundancy and double key situations were constructed so that a range officer or the commander of a missile base could not take nuclear policy into his own hands. I don’t know of a time in world history when field commanders had less autonomy. At least that was the premise of those who said that all power was in the hands of the President, only he having access to the nuclear codes that were carried in “the football” that accompanied him everywhere. The real problem, time showed, was the opposite one: officers might be reluctant to fire off their nuclear missiles even if they were given the orders to do so. And, according to his confidantes, Kennedy felt the same way: he would never consent to launching a full scale nuclear attack.
The terms under which a field commander was controlled from the Pentagon evolved by the time of the Vietnam War, but the strain between the two sides did not disappear. Now, the two sides negotiated about information. Westmoreland and his associates had learned that MacNamara’s Pentagon was interested in quantifiable information and so had learned to state their case that they were winning the war by supplying information that would demonstrate that, even if journalists and civilian experts visiting the war zone were coming up with a very different picture. Westmoreland’s great mistake after the Tet offensive, which he properly claimed had destroyed the Vietcong, was to ask for a great increase in the number of troops to be sent to Vietnam. That was somewhat contradictory in that if Tet had been such a great victory for our side, he did not need so many more troops unless it was because his real opposition, the North Vietnamese Army, was just getting into the field. Washington exerted its real authority by replacing Westmoreland as field commander because replacement rather than new instructions, which is what you would issue to a diplomat or a plant manager, is what it always comes down to.
The overall lesson of this mini-history is that you need sane people in Washington applying political judgment to military problems, which is what FDR did and what LBJ did not do, but it is difficult when your Vice President and your Secretary of Defense go rogue, as happened in the runup to the Iraq War and the nation is without a President, at the time, who can provide overall wisdom to guide them. This is turning things topsy turvey and continues to be the case today when it is military men in civilian positions who stand between the President and reckless adventures. It was not that long ago that we relied on Barack Obama to resist the advice of his own cabinet and his generals and so not get us involved in a major war in Syria.