In February of 2018, Francis Cissna, the director of Citizenship and Immigration Services (U.S.C.I.S.), announced that he was changing the agency’s mission statement. It would no longer make reference to “America’s promise as a nation of immigrants,” Cissna said. Instead, it would emphasize the agency’s charge to preserve “the nation’s lawful immigration system.” U.S.C.I.S., which is generally overshadowed by ICE and Border Patrol, its counterpart agencies at the Department of Homeland Security, processes applications for visas, citizenship, legal residency, and asylum, and has authority over a vast network of regulations and federal rules that affect the lives of tens of millions of immigrants in the United States. Cissna has been the head of U.S.C.I.S for nearly two years, and during that time he has reshaped the legal immigration system in ways that are often too technical to capture mainstream attention: he has created a denaturalization task force to strip people of their citizenship, restricted work authorizations, blocked paths for immigrants to obtain legal status, and imposed rules to penalize immigrants for their use of public services. He has also been a vocal critic of the country’s family-based legal-immigration system, which he, like the President, has long wanted to overhaul. A profile in Politico called Cissna “the man behind Trump’s invisible wall.”

One thing Cissna has always made clear is that his actions are a matter of law, not ideology. The distinction has been a point of pride for him. “Cissna is an immigration nerd,” one of his former U.S.C.I.S. colleagues told me. Any time Cissna testifies in front of Congress or leads internal agency meetings, he carries a soft-bound volume of the Immigration and Nationality Act—as if he were the embodiment of the by-the-book bureaucrat. The appearance of such fastidiousness is part of his power; his policy positions, however extreme, are shrouded in legal rationales. His denaturalization task force, for instance, was about “denaturalizing people who should not have been naturalized in the first place,” he said. “We finally have a process in place to get to the bottom of all these bad cases.” Family separation at the border came down to the fact that there needed to be “consequences” when someone crossed the border illegally. “I just feel a strong commitment to the law, and to the rule of law,” he said. “None of the things we’re doing . . . are guided by any kind of malevolent intent.”

The Trump Administration has enacted the harshest immigration policies in a century, but it has also reached the outer limits of what any government can legally do to stem immigration. The courts have blocked a number of the President’s signature initiatives, from his family-separation policy to a Presidential proclamation banning asylum. In response, the Administration has planned more aggressive enforcement measures, but none of them has changed regional migration flows. Last month, nearly a hundred thousand Central Americans were apprehended at the southern border, and the number of families seeking asylum has reached the highest level in at least a decade. Donald Trump has since vowed to cut all aid to the region, which will only worsen the conditions on the ground in Guatemala, Honduras, and El Salvador. And, at the instigation of his senior policy adviser, Stephen Miller, he has begun to purge the upper echelons of D.H.S. Last week, Trump fired his Secretary of Homeland Security, Kirstjen Nielsen, and was also preparing to oust Cissna, who has repeatedly clashed with Miller.

The main sources of contention in the Trump White House are the laws and regulations that dictate how the government must treat asylum seekers at the border. By law, migrants are given an initial screening, known as a credible-fear interview, to determine whether they are in danger in their home countries; if they pass, which a majority of asylum seekers do, border authorities allow them into the country pending a full hearing before an immigration judge. At one D.H.S. meeting last year, Miller accused department officials of sympathizing with asylum seekers, because so many migrants were passing these screenings. “You need to tighten up,” he said, according to the Times. Cissna told Miller to “stand down.”

A D.H.S. official told me, “Credible-fear hearings are the potential choke point. Most people make it through that initial round, and U.S.C.I.S. does not choke them off there.” The official added, “The credible-fear interview is the ‘asylum loophole.’ ” Closing it, however, would require changes to asylum law, and the Trump Administration is unwilling to go to Congress. Instead, the White House has directed the department to take radical action and deal with the consequences later. In the general counsel’s office at D.H.S., the official told me, “there’s a view now that we should be pushing the legal limits,” because “we’re going to win at the Supreme Court.”

Late last week, I spoke to one Administration official who had worked on the President’s 2016 campaign and continued to describe himself as a committed Trumpist. “There’s just been an atmosphere where people are not trying to implement the President’s agenda,” he told me. “A court would say, ‘You’re enjoined’ ”—legally required to stop—“and that would be the end of the discussion from the lawyers, almost like they were relieved they didn’t have to deal with it anymore.” Cissna, the official told me, “wasn’t moving fast enough, the regulations weren’t moving fast enough.” It wasn’t clear what this meant, exactly, short of Cissna rewriting asylum law in contravention of Congress. When I asked a former White House official whether such criticisms of Cissna were fair, the former official replied, “He acts like an establishment bean counter,” adding that “he is not big-think enough.”

On Tuesday, the Attorney General, William Barr, issued an order that could force tens of thousands of asylum seekers to remain in jail indefinitely while they wait for an immigration judge to rule on their claims. There is a current backlog of approximately a million cases in the country’s immigration courts, and it can take several months, if not years, for asylum seekers to get a full hearing before a judge. Trump wants his Administration to detain as many asylum seekers as it can, rather than release them with a future court date. Barr’s order relies on an obscure authority to refer immigration cases to himself for review, but it will not affect unaccompanied children or families, who remain exempt because of existing legal protections, even though they currently make up more than half of all asylum seekers at the border. “This decision might actually be the most Barr can do related to the President’s immediate concerns at the southern border,” Sarah Pierce, an analyst at the Migration Policy Institute, told me. “The President wants to be able to deport unaccompanied children and detain families. His inability to do either of these things is so well spelled-out in statute and litigation that I cannot imagine a way for the Administration to get around those using referral and review.”

Cissna, for the time being, has retained his post. Influential anti-immigration groups protested his potential firing, and Republican hard-liners in Congress came to his defense. Chuck Grassley, who once worked with Cissna on the Senate Judiciary Committee, said that the President was “pulling the rug out from the very people that are trying to help him accomplish his goal.” Cissna’s immediate boss, the new acting head of D.H.S., Kevin McAleenan, is widely seen as a relative moderate with solid credentials. He replaced Nielsen because she did not have a deputy to assume the position in her absence, but, as the official who worked on Trump’s campaign told me, “It’s well known that McAleenan is an anti-Trumper.”

Amid the staffing shakeup last week, D.H.S. officials scrambled to make preparations for a new set of rules requested by the White House, concerning immigrants who overstay their visas after entering the United States. Countries with high rates of immigrant overstays, as they’re known, would be penalized; the U.S. government would restrict visas to their citizens unless the overstay numbers declined. Then the plans were delayed. “It was a fire drill,” the D.H.S. official told me. “Chaos is at a new and higher level.” The countries at issue were African nations, such as Nigeria, Chad, and Liberia. “This has literally nothing to do with what’s going on at the border,” the official said. “The people around the President are just looking for something to do.”