Retired four-star Marine general John Kelly was sworn in as President Donald Trump's new chief of staff on Monday, moving over from heading up the Department of Homeland Security.

"At Homeland, what he has done has been nothing short of miraculous," Trump said during a Cabinet meeting Monday morning. The president was referencing government statistics showing that border crossings between Mexico and the United States have declined significantly since he assumed office in January.

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There are two theories as to why Trump may have some success with Kelly as his chief of staff. Kelly is 64, just seven years younger than Trump; former chief of staff Reince Priebus was 45. The smaller age gap meant that Kelly is Trump's "generational peer," as Kellyanne Conway told the New York Times. Kelly's military background is something the president crowed about, while Priebus was a former Republican National Committee chairman.

Despite those advantages, Kelly will still having to deal with the fact that Donald Trump is his boss. And as the past six months have shown, the president has no problem frequently straying from the carefully written scripts that his staffers would like him to follow.

During the final weeks of his tenure, Priebus and the White House communications shop had tried to better shape the news by designating formal "theme weeks" where the administration was supposed to focus its energies on emphasizing certain policy areas. The effort never worked, because Trump refused to play along with the plans he'd surely approved. But even aside from the president, the White House staff and the congressional GOP very frequently were also pushing conflicting messages or really none at all.

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The administration's first attempt at a theme — "infrastructure week" — ended up a dud when the White House failed to present an actual plan for rebuilding the nation's transportation structure. The only thing the administration could come up with was a plan to privatize air traffic control systems, which members of both parties quickly denounced. Trump also distracted from the message by getting into a Twitter fight with the mayor of London after he misunderstood something that the British politician had said.

While Trump has made more than a few stray remarks while delivering speeches, his main vehicle for stepping on his planned messages has been his Twitter account. Safely ensconced in a digital cocoon late at night and early in the morning, the president seems to enjoy lashing out at people who angered him during the hours before, no matter how much trouble these outbursts cause him. As a candidate, Trump also somewhat regularly would retweet white nationalists who followed him.

Thus far, no one in the White House has been able to do anything about Trump's Twitter addiction. During his first day on the job, communications director Anthony Scaramucci effectively surrendered to the president's wishes with respect to Twitter.

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“To me, I think it’s been very effective use of reaching the American public directly and so listen, I welcome him continuing to do that," Scaramucci said. "I think it’s very, very important for him to express his identity."

It's not clear what concessions Kelly managed to get out of Trump before agreeing to become chief of staff. Reportedly, the president had been asking him since the middle of May to take the job.

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Taking away the president's phone, restricting his access to "Fox and Friends," and getting him to read more policy papers instead of printouts from conspiracy websites would move Trump a long way forward to becoming presidential. But don't count on any of that happening.