A third senior Pentagon official has quit following Donald Trump's announcement that US forces would leave Syria.
Department of defence chief of staff Kevin Sweeney resigned after two years in the post.
He walks out just weeks after defence secretary James Mattis announced he was quitting.
Rear Admiral Sweeney said in a statement that "the time is right to return to the private sector".
His resignation statement pointedly makes no mention of Mr Trump and says it had been an "an honour to serve" alongside his colleagues in the department.
General Mattis' departure was sped up, with the senior figure exiting two months earlier than planned reportedly because of Mr Trump's anger over the extensive coverage given to Mattis' resignation.
Brett McGurk, the presidential special envoy to the global coalition fighting so-called Islamic State, also quit following the Syria announcement, as did department spokeswoman Dana White.
Sky News' diplomatic editor Dominic Waghorn explains that Mr Trump is rapidly losing his support system.
He said: "You're looking at a president who is temperamentally unsuitable to be in the job, and it's been a roller coaster ride so far but he has had stabilisers on him in the form of these former generals – (James) Mattis, (John) Kelly, (HR) McMaster.
"He's now trying to buy not so much 'yes' men but people who do his bidding.
"They are intelligent people, like (Mike) Pompeo, (John) Bolton and others but they don't dissent.
"What's really important to say is every presidency so far – if you think about (Barack) Obama and the financial meltdown, George W Bush and 9/11, his father with the invasion of Kuwait – every president does face a major crisis, a massive global crisis that they have to deal with and we've not had one with Trump so far."
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He added: "When it does, as it surely will at some point, how is he going to cope without those kind of strengths?"
No timetable has been offered so far for when US troops will be pulled out of the region, a move Mr Trump announced in mid-December.