Interesting that 7 high ranking Pentagon officials have resigned since the beginning of December. I mean, It's not like the Pentagon could do with being fully staffed at the moment or anything given the state of play in the Middle East.
I'm willing to bet that those who have jumped have seen the writing on the wall over the Ukraine scandal. It is now looking increasingly certain that VP Pence was in on it, so that also puts his staff in the frame.
If both Pence and Trump are implicated and removed before the election (highly unlikely, unless something significant drops), then Pelosi would take over as interim president, as I understand the line of succession.