This story was updated at 9:53 a.m. on March 24, 2018.

As the news leaked last week that Lt. Gen. H.R. McMaster would soon be replaced as President Trump’s national security adviser, speculation swirled about whether the storied military officer would be given a four-star command somewhere, a soft-landing after a hard run at the White House.

Instead, the only active-duty service member in the president’s Cabinet is ending his distinguished 34-year Army career with an inglorious firing.

“I am requesting retirement from the U.S. Army effective this summer after which I will leave public service,” McMaster said in a farewell statement released by the White House.

So why no fourth star after a stellar career? Theories abound, based on conversations with people at the Pentagon on Friday.

He didn’t want it. Unlike other members of Trump’s Cabinet, McMaster had little choice but to join the president’s inner circle when “invited.”

As a serving military officer he couldn’t very well refuse an assignment from his commander in chief, and it may be that after a year, he’s had enough.

Thirty-four years is a good career by any measure, and like other celebrated military officers before him, McMaster is well-positioned to pursue more remunerative options in the world of private consulting and corporate boards.

McMaster, whose differences with Trump are no secret, may also be figuring that if he were to end up, for example, as a four-star commander in South Korea, he might get an order from the president one day that he would be less than enthusiastic about carrying out.

Trump was pissed. Trump had warm words for McMaster, who he said “has done an outstanding job” and “will always remain my friend.”

But McMaster, who according to people who have served with him, has a prickly side and occasionally explosive temper.

While the White House did not blame him for the embarrassing leak that revealed Trump’s failure to follow the advice not to congratulate Russian President Vladimir Putin on his re-election, the leak appeared to come from his staff.

It might have been the last straw for a president who is fed up with people around him seeking to undermine his policy decisions. Trump may not have seen McMaster as worthy of a promotion.

There was no place for him to go. Even if Trump was of the mind to reward McMaster with a fourth star, there simply may have been no jobs available. There are only so many four-star billets in the military, most are combatant commands or members of the Joint Chiefs and there may have been no place to send him without pushing someone else aside unfairly.

The U.S. military has an “up or out” policy that dictates if officers are not promoted, they must retire. And there are fewer positions the higher you get. So, no matter how stellar the service record, there are not enough one-star general slots for all the colonels, not enough four-star slots for all the three-stars.

There’s no indication that McMaster was in line for promotion before he was tapped to be Trump’s national security advisor, so you can’t really say he was passed over.

The Army was over him. McMaster has had a celebrated career and gained considerable notoriety as a junior officer when he turned his doctoral thesis into the seminal book, Dereliction of Duty, a treatise on the failure of senior officers to “speak truth to power” during the Vietnam War that became required war college reading.

But McMaster made his share of enemies coming up the ranks, and only got his first star after the personal intervention of Gen. David Petraeus, who was heading the Army promotion board in 2008.

McMaster, who had been a senior adviser to Petraeus, had failed twice before to make the one-star promotion list.

And there was some grumbling among some serving and retired senior military officers last year when McMaster defended Trump against charges that the president improperly shared classified information with Russian diplomats in the Oval Office.

To many, that undercut the bedrock principle that uniformed military officers remain steadfastly apolitical. So there may have been little enthusiasm in the service to make a spot for McMaster.

Editor's Note: An earlier version of this article incorrectly stated that McMaster's book, Dereliction of Duty, stemmed from his master's thesis.