US President Donald Trump has announced his choice for defence secretary has withdrawn, shaking up the Pentagon at a time of rising Middle East tensions.
He tweeted that Patrick Shanahan has pulled out of consideration "so that he can devote more time to his family".
Mr Shanahan, who will be replaced by Army Secretary Mark Esper, had not yet been nominated for the cabinet post.
It comes as Mr Shanahan publicly addressed allegations of domestic violence.
Mr Trump told reporters Mr Shanahan submitted his withdrawal on Tuesday morning.
"I didn't ask him to withdraw, but he walked in this morning and said it was going to be a rough time for him," Mr Trump said, adding that he first heard of the "unfortunate" allegations on Monday.
Earlier on Tuesday, USA Today reported the FBI was investigating a 2010 incident in which Mr Shanahan and his ex-wife accused each other of assault.
Minutes after Mr Trump's announcement, the Washington Post published an interview with Mr Shanahan in which he discussed a 2011 incident when his son, then 17, reportedly beat his mother with a baseball bat, fracturing her skull.
Acting Secretary of Defense Patrick Shanahan, who has done a wonderful job, has decided not to go forward with his confirmation process so that he can devote more time to his family....— Donald J. Trump (@realDonaldTrump) June 18, 2019
....I thank Pat for his outstanding service and will be naming Secretary of the Army, Mark Esper, to be the new Acting Secretary of Defense. I know Mark, and have no doubt he will do a fantastic job!— Donald J. Trump (@realDonaldTrump) June 18, 2019
In a statement announcing his resignation, Mr Shanahan called it "unfortunate that a painful and deeply personal family situation from long ago is being dredged up and painted in an incomplete and therefore misleading way".
He said he did not want his three children "to relive a traumatic chapter in our family's life and reopen wounds we have worked years to heal".
Mr Shanahan added: "I would welcome the opportunity to be secretary of defense, but not at the expense of being a good father."
On Monday, he told the Washington Post that he regretted a legal memo he penned shortly after the alleged attack on his ex-wife.
He wrote that the "use of a baseball bat in self-defence will likely be viewed as an imbalance of force" and that "Will's mother harassed him for nearly three hours before the incident".
Before joining the Trump White House, Mr Shanahan had worked as an executive at Boeing, the US aerospace company which holds many US defence department contracts.
He took over six months ago after the resignation of former Defence Secretary James Mattis.
A leaderless Pentagon continues
Patrick Shanahan's decision will extend what is already the longest period of time the US has been without a confirmed secretary of defence.
Under his watch, the Pentagon has begun a build-up of troops in the Middle East in response to tensions with Iran. That will play out as planned, executed by the chain of military command, but still without the political weight of a senior cabinet defence official to guide developing policy and argue the Pentagon's corner at the White House.
Mr Shanahan's critics grumbled that he had ceded too much authority to the White House, that it was the hawkish National Security Advisor John Bolton who was driving policy.
His successor as acting chief of the Pentagon is the Army Secretary Mark Esper. A military analyst, Brigadier General Mark Kimmitt, told the BBC that Mr Esper was a West Point classmate of the Secretary of State, Mike Pompeo, and may share the hardline views of Mr Pompeo and Mr Bolton.
Whatever the case, the administration has made clear that President Trump does not want a war in the Middle East, and the troop deployments so far have been relatively small, aimed at protecting US forces and sending a message of deterrence to Iran.
Who will be the next Pentagon chief?
Mr Shanahan's tenure saw the longest ever period that the US has been without a permanent secretary of defence.
Mark Esper, the current Army secretary, is due to take over in an acting capacity until a new Pentagon chief is nominated.
He was Mr Trump's third choice for the role of Army secretary, after the president's first two nominees - Wall Street billionaire Vincent Viola and Tennessee state Senator Mark Green - withdrew following scrutiny over business ties and controversial comments, respectively.
Mr Esper, a 56-year-old Harvard graduate and retired Army infantry officer who served during the US war with Iraq, has been Army secretary since 2017.
He also previously worked as the former vice president of government relations at Raytheon, one of the world's largest arms manufacturers.
Mr Trump had previously voiced his intention to nominate Mr Shanahan to the post, leading to many questions about who his next choice will be.
The defence secretary must be confirmed by the US Senate.
Senate Republican leader Mitch McConnell told reporters on Tuesday Mr Esper was "a good choice for acting" secretary, but that it would be "better to have a confirmed secretary of defence".
His Democratic counterpart Chuck Schumer meanwhile said it was "appalling" to have no confirmed defence secretary at this time.
"They have so many empty positions, rotating-revolving doors in the most sensitive of security positions."
The Republican chairman of the Armed Services Committee concurred, saying in a statement: "For the sake of our national security, we need a confirmed Secretary of Defence - not just an acting. I hope we can get to that point as quickly as possible."
The announcement comes amid renewed tensions between the US and Iran after Washington accused the Islamic republic of attacks last week on two oil tankers in the Gulf of Oman.
Mr Shanahan on Monday announced the deployment of 1,000 US troops to the Middle East to counter "hostile behaviour" by Iranian forces.
He added that the US does "not seek conflict with Iran" and the action had been taken to "ensure the safety and welfare of our military personnel working throughout the region to protect our national interests".
Mr Trump on Tuesday said the US was "very well set" when it came to Iran, adding: "We'll see what happens."
Speaking on the senate floor earlier, Mr Schumer criticised the administration's lack of transparency on Iran, saying the White House "has an obligation to explain to the American people exactly what is happening and why" as US troops are once again sent to the Middle East.
"The president has yet to articulate a strategy about American involvement in the region," Mr Schumer said. "All we have heard is the secretary of state, our chief diplomat, rattling the sabres of war."