San Diego – Kirstjen Nielsen and Donald Trump were a bad fit from the start.The only question was who was going to leave whom.

The story released for public consumption is that, following a White House meeting with Trump on Sunday, Nielsen decided to resign. By all accounts, however, she wasn’t ready to call it quits. She was fired as head of the gargantuan Department of Homeland Security, which employs a staggering 240,000 people.

Late Sunday, Nielsen tweeted that she would stay on until Wednesday to help with the transition – and, no doubt, attempt to save a smidgen of face. On Monday, she met briefly with a gaggle of reporters outside her home in Alexandria, Virginia. Before brushing them off and hustling back inside, Nielsen insisted that she and Trump still had the same goal: stronger border security.

That doesn’t mean anything. Trump and Nielsen had radically different opinions of what the U.S. government – and specifically, DHS – could do to secure the border to the point where people don’t cross it without an invitation.

The media never understood the relationship between Trump and Nielsen. When it wasn’t portraying her as loyal to a fault in carrying out Trump’s policy of separating immigrant families, it was playing up the friction between the two and attributing it to the assumption that Nielsen – who seems plenty tough to most of us — wasn’t tough enough for Trump’s taste.

That’s wrong. The split between the two was never about toughness, but about what can and what can’t be done to stop something that has been occurring for as long as human civilization: the movement of people across borders in search of something better.

You can build a moat, a wall, or a blockade, but human nature doesn’t succumb to physical barriers.

For years, scores of Border Patrol agents have told me – on my own trips to the U.S.-Mexico border in Texas, Arizona and California – that there is no wall high enough, deep enough, or long enough to stop someone who has to feed his or her family. The experts understand this fact.

The split between Trump and Nielsen was about the difference between politicians who win over voters by promising the impossible, and the Cabinet officials who have the misfortune of being tasked with making the impossible happen.

In her public comments over the last few months, Nielsen dropped hints that – even if she wanted to shut off all illegal immigration -- her hands were tied by the law and the Constitution, as well as by what Congress had already authorized, and could be expected to authorize going forward.

But, in truth, Nielsen was also limited by something bigger. She was constrained by her attempt to do her job in a way that was – to use words that probably seem foreign to anti-immigrant crusader and White House Senior Adviser Stephen Miller, who reportedly had a hand in her firing – honest, practical, and realistic.

We still don’t know the specifics of exactly what occurred at Nielsen’s meeting with Trump. Did the president call her to the White House with the intention of making good on his threat to fire her, which he has been toying with since last Spring when John Kelly, then-White House Chief of Staff, reportedly intervened to save the job of his protégé? Or did matters come to a head at the meeting, over the question of whether DHS can actually stop illegal immigration?

What we know is that Nielsen’s dismissal came just a few days after she accompanied Trump on a trip to the U.S.-Mexico border to talk about immigration – a visit that, in hindsight, might have been designed to show the world how little Trump understands about either the border or immigration. Trump – who used the visit to hang a No Vacancy sign and announce that “the country is full”— seems to really believe that it’s possible to “shut down” a 2,000-mile border and stop illegal immigrants from entering the United States.

Consider what happened when Trump — during a recent White House meeting — ordered Nielsen to close down the very busy port of El Paso and to do it the very next day. Nielsen was the voice of sanity. According to CNN, she told Trump that would be a bad and even dangerous idea.

According to two people in the room, Trump said: "I don't care."

Who thinks this way? I’ll tell you who. People who don’t live near the border and who only visit now and then for a few hours at a time. Also, people who don’t work for Immigration and Customs Enforcement, Border Patrol or DHS, which oversees them both. In other words, the further you are from the problem, the more likely you are to think you have the solution.

Which brings me to how Trump and Nielsen got off to such a bad start in the first place, and why the relationship was always doomed to fail.

I’m tempted to say that trouble began in December 2017, during Nielsen’s confirmation hearings before the Senate Judiciary Committee. At one point, the nominee was asked if she agreed with the statement that DHS was “not going to build a wall from sea to shining sea” along the U.S.-Mexico border. She responded with a simple, one-word answer: "yes."

That response could not have set well with Trump who, in courting his supporters, oversold the idea of a “big, beautiful” border wall, 12 to 15 feet high and 2,000 miles long – and one that Mexico was going to pay for no less.

But, it’s more accurate to say that the seeds of what would grow into this dysfunctional relationship were planted a year earlier, way back in December 2016 when Trump chose Kelly – a retired Marine general – to head DHS. Nielsen was his deputy. No Kelly, no Nielsen.

It was Kelly who made the statement Nielsen agreed to at her confirmation, about how DHS was “not going to build a wall from sea to shining sea” on the U.S.-Mexico border. Now, to be fair, Kelly also told the senators that he agreed that “ physical barriers work” in combating illegal activity, but he emphasized that these barriers should be placed strategically where they will do the most good.

An answer like that makes sense. It’s honest, practical, and realistic. Which explains why Trump, and Miller, wanted nothing to do with it. They prefer an immigration policy that is simplistic, showy, and demagogic.

All of this helps makes clear what sort of candidate should toss their hat into the ring to replace Nielsen. Sycophants preferred. No grownups need apply. Grasp of reality, optional.