Does the President of the United States have the ability to deploy nuclear weapons at will?

Question

I heard this claim back during the Bush years, and recently I've been hearing it again. For example:

Donald Trump’s Ghostwriter Tells All - The New Yorker

I genuinely believe that if Trump wins and gets the nuclear codes there is an excellent possibility it will lead to the end of civilization.

Sometimes, a "button" is used metaphorically. Hillary Clinton refers to a button as well.

Is there some mechanism through which the President of the United States can deploy nuclear weapons without anyone else's approval?

Answer 1

According to retired Lt. General Mark Hertling, who personally participated in such drills, and wrote Nuclear Codes: The President's Awesome Power

That bag -- carried by the military aide -- has been within feet of the commander in chief ever since for any situation where the president believes the use of nuclear weapons is warranted. If that is the case, he is able to order the military aide to open the briefcase and issue an alert to the Joint Chiefs of Staff. While that is occurring, the president reviews options from the nuclear triad -- submarine launched missiles, aircraft with atomic weapons, or land-based intercontinental ballistic missiles (ICBMS) -- and then decides on a course of action.

The aide then connects the president with the National Military Command Center (NMCC) -- in the Pentagon or an airborne command and control element -- and positively identifies himself with a special code issued on a plastic card. Most presidents have kept that card -- called the "biscuit" -- in their possession at all times.

Should this happen, the code on the president's card would be confirmed by either the secretary of defense, or the watch officer (a general or admiral on duty) at the NMCC, and the president could then order a strike. The president always has the authority to order an attack, with his options ranging from the launch of one missile to extensive, massive strikes from one or several elements of the triad: bombers, submarines, missiles.

So, according to General Hertling, while it must be confirmed by a general, admiral or the secretary of defense that it is really the president giving the order, "The president always has the authority to order an attack".

For more information see Nuclear Command and Control in NATO: Nuclear Weapons Operations and the Strategy of Flexible Response. I'll only quote a portion:

Although the President had the sole authority and responsibility to order the use of nuclear weapons the control of nuclear weapons operations was exercised by the NCA [National Command Authorities] which, according to the 1971 Department of Defense Directive 5100.30, consisted only of 'the President and the Secretary of Defense or their duly deputised alternates or successors'. The NCA could also be widened to include the most senior US military officer the Chairman of the Joint Chiefs of Staff (CJCS). The sharing of operational control – if not authority – enabled even this most senior level of decision-making to meet the 'two-person' rule which governs all US nuclear weapons activities. This rule stated that all decisions, procedures or processes involving nuclear weapons had to be carried out by at least two individuals, and was intended as a hedge against unauthorised or irrational action by anyone in the command chain.

See also Department of Defense directive NUMBER 3150.02:

The President, as Commander in Chief of the Armed Forces, is the sole authority for the employment of U.S. nuclear weapons.

In Conclusion, physically the president alone can not use nuclear weapons; the involvement of other people is absolutely required. However, if the chain of command is followed, policy is that the president is the sole authority for use of nuclear weapons.

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For historical comparison, according to the New World Encylopedia:

The Washington Post [reference 21] reported that many Europeans and leaders around the world thought that Reagan was "a cowboy" and "scary." Carter's campaign implied that Reagan was "a trigger happy cowboy."[22] The Iranian hostage-takers in particular reported being unsure of what Reagan would do.[23] Iranian uncertainty about Reagan's plans may have been the main motivation behind the timing of the release of the hostages

Answer 2

Essentially, yes.

The decision is technically solely up to the President. Of course, he does not personally perform the launch; there are other people in the chain of command following his orders.

The International Commission on Nuclear Non-proliferation and Disarmament (ICNND) studied the process in depth. (They were searching for possible cyber-attack vulnerabilities.)

The most portable way the president has to command nuclear strikes is the "nuclear football".

It is a specially outfitted briefcase which can be used by the President to authorize a nuclear strike when away from fixed command centres. The President is accompanied by an aide carrying the nuclear football at all times...

The attack options provided in the football include single ICBM launches and large scale predetermined scenarios as part of the Single Integrated Operational Plan. Before initiating a launch the President must be positively identified using a special code on a plastic card, sometimes referred to as ‘the gold codes’ or ‘the biscuit’. The order must also be approved by a second member of the government as per the two-man rule (Pike 2006).

This two-man rule is officially described in Air Force Instruction 91-104. Basically, two people have to validate any nuclear order. Again, from the ICNND,

The US uses the two-man rule to achieve a higher level of security in nuclear affairs. Under this rule two authorized personnel must be present and in agreement during critical stages of nuclear command and control. The President must jointly issue a launch order with the Secretary of Defense; Minuteman missile operators must agree that the launch order is valid; and on a submarine, both the commanding officer and executive officer must agree that the order to launch is valid.

However, the two-man rule from AFI 91-104 is only meant to confirm the command is valid; it's not meant to decide whether or not a command should be issued. Ultimately, that decision rests solely upon the President of the United States (or his successor). The Secretary of Defense (or his successor) simply affirms that the command was indeed issued, and done so by the appropriate person.

For succession purposes, the codes are actually distributed to at least the President and Vice President, though the VP's commands would only be listened to if the President were unreachable.

Why Clinton's Losing the Nuclear Biscuit Was Really, Really Bad

A nuclear launch order can only be commanded by the President, or his successor in case of death or removal from office.

In effect, without Clinton's "biscuit," as the personal identifier is called, the President would not have been able to initiate a launch order or confirm a launch order executed by someone else...

So what happens if the President doesn't have his identifier?

The commander in chief of NORAD resorts to the next person the NCA list, the Vice President.

...

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This shouldn't be very surprising; the military has always worked with a chain of command, with a single person ultimately responsible at each level. There's no democracy, no voting.

Naturally, it possible someone could question the order, but the further from the President, the less information they would have.

It might seem like an insane process, but remember that in a real nuclear event, every second would count as the US and its enemy try to remove as much of each other's nuclear capacity as possible.

So, terrifying...but that's nukes.

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EDIT 1: To use an analogy from the comments to explain the role of the SecDef:

it's like a judge performing a marriage. They don't decide who marries whom when; they simply confirm that the decision is indeed being made by the appropriate parties

The Security of Defense (technically) doesn't second guess the wisdom of the decision. And even if he did, it would be potentially useless check as the President appoints and dismisses the Secretary of Defense (and changes the order of succession at will).

This answer describes the official process. As with any protocol that involves a human, it's conceivable that it could be broken, but that speculative and currently without precedent. I believe the question assumes adherence to policy, otherwise the answer is false from outset.

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EDIT 2: A recent (Dec 2018) opinion article from The Washington Post agrees with this assessment.

The secretary of defense has no legal position in the nuclear chain of command, and any attempts by a secretary of defense to prevent the president from exercising the authority to use nuclear weapons would be undemocratic and illegal. With or without Mattis, the president has unchecked and complete authority to launch nuclear weapons based on his sole discretion.

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EDIT 3: Expert Franklin Miller has said the same: that the President has essentially unilateral power to launch nukes.

The only authoritative way to change that is to change his status as President: removal by Congress, or the VP and a majority of the Cabinet declaring him disabled/unfit (25th Amendment).