Twelve

At Brookings, Michael O'Hanlon writes:

A president of the United States can, in theory, launch nuclear war by personal decision—without any checks or balances. Whether we really think any of the candidates for president in 2016 would cavalierly start a nuclear war, the bombastic and bizarre character of much of this year’s electoral debate should make us take this question seriously. Someday, the United States really could have a mentally ill president who chose to do the unthinkable. The odds are low, but we should seek to make them even lower, given the stakes at hand. Because it looks like humankind will be stuck with the nuclear bomb for many decades (if not centuries) to come, moreover, the solution to this problem cannot be simply to get rid of all existing nuclear arsenals. We need a more immediate answer.

To be sure, a president is required by the War Powers Act of 1973 to seek congressional approval for any military action within 60 days of its inception. But most presidents consider that act unconstitutional. In any event, a nuclear war could easily devastate the planet within just days or hours—long before the 60-day stipulation would be binding. Even if a president had obtained congressional approval for a war that began using only conventional weapons, no provisions of the War Powers Act would require subsequent congressional action prior to nuclear escalation.

In short: A president could push the button all by himself or herself, legally- and constitutionally-speaking. Physically, military personnel would need to carry out the strike of course. They could choose not to, perhaps at the instruction of the secretary of defense or the four-star officer leading Strategic Command—who together constitute the chain of command between the president and the trigger-pullers. But any military officer ignoring a presidential order would be in open insubordination, subject to dismissal and court martial.

Only twelve Americans have ever held this power:

1. Truman
2. Eisenhower
3. Kennedy
4. Johnson
5. Nixon
6. Ford
7. Carter
8. Reagan
9. GHW Bush
10. Clinton
11. GW Bush
12. Obama 

Every one had flaws and made mistakes.  Some did very bad things.  But all twelve had the good sense not to launch a nuclear attack.  Will our luck run out with number 13?