DOD is 6th in line.
Wow that has changed a lot, when I was a young Marine it was US President then Secretary of Defense.
I believe it still is. President to Secretary of Defense (Homeland Security for Coast Guard) to appropriate unified combatant command and down the ranks. He’s number 6 in order of succession after VP, House Speaker, Senate Pro Tempore, State, Treasury then Defense. All Democrats except Mike Johnson.
I think 6th in line refers to presidential succession. Sec Def is 6th in line. I’m thinking you are referring to a nuclear launch order. President sends next to SecDef who confirms President’s identity but cannot veto him.
So yes, if Austin was incapacitated in a hospital and told nobody, it’s serious. It would take time to realize he was missing and go around, fire him or whatever the law requires, and use his deputy or whoever.
This article disagrees:
The Surprisingly Disorderly History of the U.S. Presidential Succession Order
“After Harry S. Truman assumed the presidency following the death of Franklin D. Roosevelt in 1945, he argued that elected officials should be higher in the succession order than appointed cabinet members. Thus another Presidential Succession Act was adopted in 1947. The speaker became second in line, the president pro tempore was third, and the secretary of state fourth...
Later notable modifications included the Twenty-fifth Amendment, which was ratified in 1967. It explicitly stated (unlike the Constitution) that if the vice president assumes the Oval Office, he or she would be the president—not the acting president...
In the ensuing years, some have pushed for additional changes....”