Introduction
There is a frequent theory, particularly on the part of the American Left, that Presidents (especially Republican Presidents) are but the minions of shadowy Secret Masters who "really" run things, and who "control" the President. This accusation was most notably made about Ronald Reagan and George W. Bush.
(for some reason Richard M. Nixon and George H. W. Bush were more commonly accused of being "secret controllers" than "secretly controlled.")
Whenever I've heard this sort of thing, I've always wondered ... "how?"
If you look at the U.S. Constitution, you will quickly note that the most powerful single individual in the American political system is the President.
He alone is in head of a whole branch of government. He can give orders to anyone in the Executive Branch, and unless these orders be blatantly illegal or unconstitutional, that person is bound in both honor and law to follow them. He can negotiate treaties with foreign Powers, subject only to the Senate's power to refuse to confirm them. He can deploy American armed forces where he will, even into action, subject only to the power of the Congress to withdraw authorization or funding of the action). He can, by himself, veto a law, and this veto can only be overridden by a two-thirds supermajority. He has a whole department (the Secret Service) one of whose primary duties is the defense of his life and the lives of his family. And those are just the ones I thought of immediately -- I've probably missed a few.
All in all, not someone who it would be easy to push around.
So how could someone else "control" him?
Persuasion
The most obvious, and probably practical method, is persusion. If you are someone who the President trusts and frequently consults for advice, then you are in a strong position to offer such advice as will incline him to or against any particular course of action.
History is full of such individuals. They have included family, friends, old business partners, government advisors, and political advisors.
Yet is this really "control?" The President is in this case free to accept or reject the advice in any given case. He can even choose to reject an advisor, if the advice proves consistently bad or distasteful. Surely, to call this "control," when it is utterly incapable of making the President do anything he really doesn't want to do, inflates the whole notion of "control" to irrelevance?
Inducements
One can also, of course, offer the President something of value. This could be as crude and illegal as a bribe, or as refined and above-board as a quid pro quo of mutual political support, or even respect. Inducements can be postive (do as I say and I will work with you) or negative (do as I say or I will work against you).
Inducements pervade politics. They are, in fact, the essence of politics -- how compromise can be achieved non-violently. Presidents frequently do, or refrain from doing things, due to inducements.
But do these constitute "control?" Perhaps in a very large and systemic sense, yes. Which is, of course, precisely what the Founders wanted --an Executive controlled by the checks and balances inherent in operating in a system in which other players also have power.
If you consider how a President uncontrolled by this sort of system might behave, you'll see why. (Don't just take my word on it. Read Roman Imperial history!).
This is not, however, "control" in the sense of "being able to tell the President what he must do." The President is the most powerful actor within this system of checks and balances, and he can strategically choose which inducements to accept and which to reject.
What's more, no one individual would be in a position to monopolize the inducements offered a President. Not even one political faction could be that powerful.
Hence inducements fail as a means of controlling the President. Once again, to call this "control" would be to inflate the term to irrelevance.
Forcible Threats
One could outright threaten the President. "Do as I say or you will die," or even "do as I say or another person who you value will die," and so forth.
This is often what the conspiracy theorists mean.
But if you look at the power of a President, there is an obvious flaw with such a plan. Namely, you are threatening a man with the whole Executive Branch at his command. Worse, by making the threat, you are giving him the legal and constitutional sanction to USE this force AGAINST YOU!!!
There are very few would-be controllers who could survive the consequences should the President take the threat seriously. Indeed, the conspiracy theorists are usually at this point forced to imagine vast, nebulous plots involving practically everyone else in the Federal Government, in order to get around this difficulty.
Not even being a foreign head of state would necessarily provide safety in this case. Saddam Hussein made such threats -- he was a foreign head of state -- and he ended his life swinging on a noose, in part because he tried to assassinate an American President. American Presidents, because of America's strength, are even less vulnerable to such threats than would be most national leaders.
In any case, a threat can't make anyone do anything, unless they care less about their freedom than about the risk of defiance. And anyone who has made it to the Presidency is not going to be an easy target for bullying -- the road is not an easy one, and cowards generally find it too hard to walk.
One could theoretically control a President through threats, but he would have to be an uncommonly weak-willed man, and one would be at great personal risk every moment that one tried to apply such control.
Force (*)
This is the obvious next step if a threat is ignored, or the first step if one logically deems that a President would respond to a threat by simply destroying the one making the threat. The obvious way to use force to prevent a President from doing something is to physically attack him, with the aim being killing or crippling to the point that he could no longer function as President.
Force runs into some of the same problems as threats. The President is well-protected, and the risk of retaliation against oneself is very great. It does have one signal advantage for the would-be controller -- namely, the President need not be aware that you are going to use force on him until one actually does so.
The problem is that killing a President does not make one President. It makes the person next in the line of succession President. So, at a minimum, that person must be either friendly to the conspiracy or at least unwilling to act against them.
This is a major problem. Vice-Presidents do not normally have murderous hatred for their Presidents. Furthermore, the Conspiracy knows a deadly secret about the new President (he was with us in the assassination conspiracy) and hence the new President may be tempted to "dispose" of his former allies. (Hey, he was willing to kill the President, why will he have qualms against killing you?)
In short, force is a possibility, but only under very specific circumstances, and probably not at all unless the Vice-President is actively in the conspiracy.
Conclusion
While a very weak-willed President might be manipulated by a person or cabal such that he was effectively under their control for many purposes, such "control" would still be at that President's discretion, and he would still have the final say in all important decisions. For this reason such "control" would be unlikely to be long-lasting. Accusations of such "control" should logically be viewed as blame-shifting, often engaged in by Presidents or their supporters as ways of "proving" that "their guy" didn't really execute some (unpopular) policy -- it was all the fault of his "controllers."
In general, the President really is the man in charge. Of the Excecutive Branch, anyway.
"God is very high, and it is very far to the Tsar." :)
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(*) Needless to say, I am very much opposed to the use of force against a President of the United States of America, and I certainly do not advocate, and I do strongly condemn, any such thing. (With Love, for Carnivore's Operators) :)