Despite the historic first, the nomination will come with some controversy. Biden‘s choice continues the presidential pattern that blurred the lines of “civilian” control of this nation’s military. Since Austin retired only four years ago, he is prohibited from holding the position without a waiver granted by Congress.
This is likely, but not certain, to be a formality. Initial Trump Secretary of Defense Jim Mattis was granted such a waiver easily, perhaps due in part to congressional relief that at least one person on Trump‘s incoming team of mostly allies and weirdos had plausible qualifications for the job they were seeking.
That said, Austin is said to be a no-bluster, low-profile commander with a strategic eye and, of course, past top-tier positions in fighting the wars the nation remains near-permanently enmeshed in. Biden presumably chose him because his ideas best mesh with Biden‘s own desired changes in military policy and strategy.