According to a report in the Washington Post, outgoing Defense Secretary James Mattis assured Congressional leaders that he would keep a close eye on President Donald Trump to keep him from starting a nuclear holocaust.
The Post reports, "Every day, the U.S. nuclear early warning system is triggered by some event or another, mostly civilian and military rocket launches by one or more of a dozen countries with ballistic missiles. When such launches appear to threaten North America, the head of U.S. Strategic Command is alerted, and sometimes these alerts warrant the urgent notification of the president."
With the knowledge that the call would go directly to the volatile Trump, Mattis reportedly made other arrangements to be brought into the loop immediately.
"For over a year, Mattis has been trying to reassure congressional leaders that he could help check some of Trump’s impulses, in part by intervening in the nuclear chain of command," the Washington Post's Bruce Blair and Jon Wolfsthal explained. "In a break with normal procedures, Mattis reportedly told the commander of the Strategic Command to keep him directly informed of any event that might lead to a nuclear alert being sent to the president. He even told the Strategic Command 'not to put on a pot of coffee without letting him know.'"
The report goes on to note that concerned lawmakers took his assurances to mean he "would either deal with a possible threat before it reached Trump or ensure he was present to advise Trump when such an alert arrived."
You can read more about Trump's access to nukes here.