Analysis by Chris Cillizza, CNN Editor-at-large

(CNN) -- Two weeks before she was fired, Kirstjen Nielsen told President Donald Trump "no."

Nielsen, at the time the head of the Department of Homeland Security, resisted Trump's order to close the US border with Mexico in El Paso; by telling Trump the move would be "a bad and even dangerous idea, and that the governor of Texas, Republican Greg Abbott, has been very supportive of the President," reported CNN's Jake Tapper on Monday.

Trump eventually scrapped the idea -- although he floated the idea of closing the entire US-Mexico border via Twitter soon after. Eventually he gave up on that idea as well.

Nielsen joined a surprisingly long list of White House aides who told Trump "no" when he asked them to do something that they believed was either illegal, politically perilous -- or both. Not coincidentally, all of the people who told Trump "no" don't work for him anymore.

My list of them is below. Did I leave anyone out? Email me at cillizza@cnn.com.

Former Secretary of State Rex Tillerson: "When the President would say, 'Here's what I want to do and here's how I want to do it.' And I'd have to say to him, 'Well Mr. President, I understand what you want to do, but you can't do it that way. It violates the law. It violates treaty.'"

Former FBI Director James Comey: "'I hope you can see your way clear to letting this go, to letting [former national security adviser Michael] Flynn go,'" Comey said Trump told him in a one-on-one meeting at the White House in 2017. "'He is a good guy. I hope you can let this go.'" Comey recounted that he told Trump only that Flynn "is a good guy" but made no promise to end the FBI investigation.

Former Defense Secretary Jim Mattis: Trump wanted Mattis to execute the withdrawal of troops from Syria. Mattis refused -- and resigned. Trump later agreed to leave 400 US troops in Syria.

Former White House chief of staff John Kelly: Kelly repeatedly clashed with Trump -- and said in an exit interview with the Los Angeles Times that his time in the job should be measured by the things he kept Trump from doing. This, from The New York Times, is enlightening in that regard:

"In the Los Angeles Times interview, Mr. Kelly conceded that Mr. Trump often pressed against the legal boundaries of his role, as the former secretary of state, Rex W. Tillerson, said recently.

"The president would ask things like, 'Why can't we do it this way?' Mr. Kelly said."

Former White House counsel Don McGahn: In June 2017, Trump ordered McGahn to fire special counsel Robert Mueller. McGahn refused and said he would quit if Trump went through with it.

Former Chief Economic Adviser Gary Cohn: Cohn was vocal in his opposition to Trump's plan to impose tariffs on steel and aluminum imports. Trump put them in place anyway. Cohn quit.

Former national security adviser H.R. McMaster: The beginning of the end for McMaster was when he told the Munich Security Conference in 2017 that, contra Trump, evidence that Russia sought to interfere in the 2016 election was "incontrovertible."Trump publicly pushed back on that contention. Less than a month later, McMaster was gone.

Former Attorney General Jeff Sessions: Sessions repeatedly refused Trump's cajoling to un-recuse himself in the Russia investigation being run by the FBI. Trump never forgave Sessions for that decision, publicly bullying him and undermining him until the President removed Sessions from the job the day after the 2018 election.

White House lawyers Ty Cobb and John Dowd: Two of the original members of Trump's legal team focused on the Mueller report, both men repeatedly urged the President not to personally attack the special counsel. Trump, obviously, ignored that advice -- and eventually got rid of both men.

The common thread here -- as you likely noticed by now -- is that a grand total of none of the people above who stood up to one of Trump's demands are employed by Trump's White House any longer. Which is, uh, not a coincidence. Trump has systematically eliminated opposition to even his wildest asks and demands over the past 18 months -- prizing unstinting loyalty to him over all other traits when it comes to searching for replacement to these "no" men and women.

The formula to get -- and keep -- your job within the Trump administration is now clear: Don't tell the President "no."