Question: Who has the power to launch nuclear weapons in the us?

Disambiguated questions and answers:
context: Unless the United States is actually under attack by a nuclear-capable adversary, the President can order nuclear strikes only in his capacity as a member of the two-man National Command Authority, the other member of which is the Secretary of Defense. Their joint decision must be transmitted to the Chairman of the Joint Chiefs of Staff, who will then direct the National Military Command Center to issue an Emergency Action Message to nuclear-capable forces.
question: Who has the power to launch nuclear weapons in the us if the United States is under attack by a nuclear-capable adversary?
short_answers: the President
President
wikipage: Nuclear weapons of the United States
context: No context provided
question: Who has the power to launch nuclear weapons in the us if the President is alive, other than the President?
short_answers: Secretary of Defense
the Secretary of Defense
context: Starting with President Eisenhower, authority to launch a full-scale nuclear attack has been delegated to theater commanders and other specific commanders if they believe it is warranted by circumstances, and are out of communication with the president or the president had been incapacitated. For example, during the Cuban Missile Crisis, on 24 October 1962, General Thomas Power, commander of the Strategic Air Command (SAC), took the country to DEFCON 2, the very precipice of full-scale nuclear war, launching the SAC bombers of the US with nuclear weapons ready to strike. Moreover, some of these commanders subdelegated to lower commanders the authority to launch nuclear weapons under similar circumstance. In fact, the nuclear weapons were not placed under locks until decades later, so pilots or individual submarine commanders had the power, but not the authority, to launch nuclear weapons entirely on their own.
question: Which general has the power to launch nuclear weapons in the us during the Cuban Missile Crisis?
short_answers: General Thomas Power
wikipage: Nuclear weapons of the United States
context: Since World War II, the President of the United States has had sole authority to launch U.S. nuclear weapons, whether as a first strike or nuclear retaliation. This arrangement was seen as necessary during the Cold War to present a credible nuclear deterrent; if an attack was detected, the United States would have only minutes to launch a counterstrike before its nuclear capability was severely damaged, or national leaders killed. If the President has been killed, command authority follows the presidential line of succession. Changes to this policy have been proposed, but currently the only way to countermand such an order before the strike was launched would be for the Vice President and the majority of the Cabinet to relieve the President under Section 4 of the Twenty-fifth Amendment to the United States Constitution.
question: Who has the power to launch nuclear weapons in the us if the President has been killed but the Vice President has not been killed?
short_answers: the Vice President
Vice President
wikipage: Nuclear weapons of the United States
context: No context provided
question: Who has the power to launch nuclear weapons in the us if the President and Vice President have been killed but the Speaker of the House of Representatives has not been killed?
short_answers: the Speaker of the House of Representatives
Speaker of the House of Representatives

Long-form answers:
knowledge: content: Command and control procedures in case of nuclear war were given by the Single Integrated Operational Plan (SIOP) until 2003, when this was superseded by Operations Plan 8044. 
wikipage: Nuclear weapons of the United States Command and control
content: Since July 2012, the US nuclear war plan has been OPLAN 8010-12
wikipage: Single Integrated Operational Plan
long_answer: Command and control procedures during a nuclear conflict are stipulated by OPLAN 8010-12 since July 2012, where it gives the power to launch nuclear weapons to a two-man National Command Authority comprised of the President and the Secretary of Defense, who must communicate their decision to the Chairman of the Joint Chiefs of Staff and the National Military Command Center to issue an Emergency Action Message to nuclear-capable forces. If the President is killed, then the power falls into the hands of the Vice President, following to the Speaker of the House of Representatives is the Vice President is also dead. Starting with President Eisenhower, authority to launch a full-scale nuclear attack has also been delegated to theater commanders and other specific commanders if they believe it is warranted by circumstances.  For example, during the Cuban Missile Crisis, General Thomas Power, commander of the SAC, took the country to DEFCON 2, the very precipice of full-scale nuclear war, launching the SAC bombers of the US with nuclear weapons ready to strike.