Army General Lloyd Austin III, commander of the US Central Command, speaks during a hearing of the Senate Armed Services Committee March 8, 2016 in Washington, DC.Brendan Smialowski | AFP | Getty ImagesWASHINGTON — President-elect Joe Biden on Tuesday defended his decision to nominate retired four-star Army Gen. Lloyd Austin to be his secretary of Defense, a personnel choice that could become one of the incoming president’s most controversial.Under the National Security Act of 1947, Congress has prohibited any individual from serving as secretary of defense within seven years of active-duty service. But Austin left the Army just four years ago, and he would require a special congressional waiver in order to bypass the seven-year rule.Writing in The Atlantic, Biden tacitly acknowledged that Austin’s nomination violates the civilian requirement, but he argued that the strength of Austin’s qualifications outweighs the potential harm of blurring the civilian-military divide.”I respect and believe in the importance of civilian control of our military and in the importance of a strong civil-military working relationship at DoD—as does Austin,” wrote Biden.”Austin also knows that the secretary of defense has a different set of responsibilities than a general officer and that the civil-military dynamic has been under great stress these past four years,” Biden wrote.If confirmed by the Senate, the 1975 graduate of West Point would be the first Black leader of the Pentagon, breaking one of the more enduring glass ceilings in the U.S. government.Commander of US Central Command Gen. Lloyd Austin III conducts a media briefing on Operation Inherent Resolve, the international military effort against (IS) Islamic State group, on October 17, 2014, at the Pentagon in Washingon, DC.Paul J. Richards | AFP | Getty ImagesAustin also has a personal relationship with Biden, having earned the president-elect’s trust and confidence for spearheading the global coalition against ISIS, which began in 2014, while Biden was vice president and Austin was leading U.S. Central Command.Biden also emphasized in his Atlantic essay that despite Austin’s recent active-duty service, he understands “that our military is only one instrument of our national security.””Keeping America strong and secure demands that we draw on all our tools,” wrote Biden. “He and I share a commitment to empowering our diplomats and development experts to lead our foreign policy, using force only as our last resort.”Nonetheless, news of Austin’s likely nomination was greeted with skepticism on Capitol Hill this week, and several key senators said they were unsure whether they would vote to grant Austin the necessary waiver to assume the position of secretary of Defense.”That’s the exception, not the rule,” Majority Whip John Thune, R-S.D., told reporters on Tuesday. “I’m not ruling it in or ruling it out. But I think it’s something we’ll have to consider when the time comes.”Montana Democratic Sen. Jon Tester also said he was reluctant to grant a waiver for Austin, even though the retired commander would make “a great secretary.””I think this guy is going to be a great secretary,” Tester told reporters. “I just think we ought to look at the rules.”Congress set aside their concerns about having a career military officer lead the Pentagon in 2016 when President Donald Trump tapped retired four-star U.S. Marine Corps Gen. Jim Mattis, who was only three years out of uniform at the time.