Joe Biden’s plan to nominate retired Gen. Lloyd Austinto serve as Defense secretary will set off an explosive fight in the Democratic Party about civilian control of the military.

Why it matters: Many Democrats have been horrified by what they saw as politicization and the erosion of civilian control of the military under President Trump, and they put the Biden team on notice that Austin wouldn't be guaranteed the congressional waiver he needs to serve.

“Waiving the law should happen no more than once in a generation,” Sen. Jack Reed, a West Point graduate and former Army Ranger, said when retired Gen. James Mattis was confirmed as Pentagon chief in 2017. “Therefore, I will not support a waiver for future nominees.”

• A person familiar with the matter said there have been "early consultations" with lawmakers on the waiver issue, and the transition is aware it's a hurdle they need to clear.

Be smart: Austin also sits on the board of defense contractor Raytheon, which could also be problematic for some Democratic senators.

The backstory: Austin, a retired four-star general who once headed Centcom, needs a waiver because he has not been out of uniform for more than seven years.

• In the 73-year history of the law requiring them, Congress has passed only two waivers.
• The Senate approved Mattis's 81-17, with 16 Democrats and Sen. Bernie Sanders voting against it.
• In the House, the vote was closer, 268-151, with only 36 Democrats supporting it.
• Sen. Kirsten Gillibrand, who voted nay, was as clear as Reed: “Civilian control of our military is a fundamental principle of American democracy, and I will not vote for an exception to this rule."

The alarm about Trump's military man-handling culminated with Trump’s walk from the White House to St. John’s Church, after the National Guard had cleared the area of protestors.

Some lawmakers had advocated for other candidates right up to the news of Austin's selection. The announcement also cheered their critics.

• House Armed Services Chair Adam Smith told reporters that Michèle Flournoy was "hands down the most qualified person" for the job. In 2017, Smith voted against the waiver for Mattis.
• Code Pink, the anti-war advocacy group, took to Twitter to claim “victory” for sinking Flournoy’s candidacy.
• It warned: “Get ready, Gen. Austin. We’re coming for you.”

The bottom line: Picking an African American to lead the Pentagon will give Biden more leeway in picking a white person to serve as his attorney general.

• Sally Yates, Sen. Doug Jones and former Supreme Court nominee Merrick Garland are all possibilities.