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This morning, a top aide to President Trump told Axios that he “doesn’t want to look weak” by abandoning his policy of family separation. Trump “feels boxed in, is frustrated and knows it’s bad politics — but also understands it’s not a fight he can back down from.”

But back down he did. In the face of overwhelming public pressure, Trump relented and agreed the policy his administration had deliberately engineered was a real policy, and that he could change it, and would.

Even a casual understanding of human psychology or American history would have made it patently obvious that the cruel method of terrorizing migrant families by wresting children from parents was bound to provoke a profound backlash. The prospect of losing a child is the most elemental fear any parent can imagine. During the 19th century, abolitionists used the breakup of enslaved families to foment disgust with the institution, calculating that even deeply racist whites would recoil.

The willingness of Trump and his administration to plunge ahead anyway with a barbaric tactic is primarily a reflection of their moral emptiness. Homeland Security Secretary Kirstjen Nielsen reportedly opposed it in private, yet energetically obfuscated on its behalf in public. Why didn’t she resign in protest? Why hasn’t anybody?

The reversal also demonstrates the comprehensive failure of Trump’s immigration agenda. Trump is facing the inevitable dilemma of a populist leader: He was elected promising easy solutions, and is discovering none exist. He has not built a wall, and Mexico has not paid for it. Instead, he is reduced to complaining about the status quo in the same terms he used as a candidate, saying things like, “We have the worst immigration laws in the entire world by far. Other countries laugh at us.” Those are the laws he promised to fix.

Trump’s internal assessment is, if anything, even more grim. “Over the past few months, Trump has called Nielsen and [Attorney General Jeff] Sessions to account as the number of immigrants flooding across the southern border has spiked. Arrests on the border nearly tripled year-over-year, rising to 40,344 this past May from 14,519 in May 2017,” reports Politico. “In private, Nielsen and Sessions have pointed the finger at each other when pressed about the rising number of illegal border crossings, but Nielsen has borne the brunt of Trump’s frustration.”

He subjected Nielsen to a tirade six weeks ago that lasted an incredible 30 minutes, and concerned the comprehensive failure of their efforts to control the border. Trump settled on family separation after every other method at his disposal collapsed. On the central policy promise he made, Trump is a flop, and he knows it.

The collapse revealed one more important thing: The power of mass revulsion. There are limits, after all, to what Trump can do without risking his patina of legitimacy as head of state. Outraged voters flooded Congress with calls, as they did to oppose the Republican plan to repeal Obamacare, and with the same success. Intense voter opposition matters. It also matters when Laura Bush speaks out against him, and when Steve Schmidt resigns from the Republican party, and when American Airlines declares it won’t cooperate with the administration. It is a sign that the peripheral elements of Trump’s coalition have a breaking point, that while they can tolerate a great deal of sub-rosa venality, visceral outrages can create a dangerous reaction.

The damage of Trump’s horrific policy cannot be fully undone. But, in the face of his every instinct, Trump was forced to retreat. He didn’t want to look weak, but he does, because he is.