General Terrence O'Shaughnessy, commander of the United States Northern Command.
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Of course, all of the following have to be incapacitated or unavailable for decision making:
1) VP
2) Speaker
3) President Pro Tempore of the Senate (Grassley)
4-17) Cabinet officials in order of creation of the position, from Secretary of State first, to Secretary of Homeland Security last...Transport Sec. Elaine Chow is ineligible to be president, and DHS acting secretary may or may not be eligible...he may have to slug it out with General O'Shaughnessy)
I presume if O'Shaughnessy is incapacitated, the next in line for command of the Northern Command becomes devolved effective president.
Interesting that the Supreme Court is excluded from the chain of succession.
The military is only supposed to take over temporarily, until a new leader can be elected.
Link: General Terrence O'Shaughnessy