Shortly before Christmas last year, Susan Rice and Jake Sullivan, two top advisers to President-elect Joe Biden, sat for an interview with EFE, a Spanish wire service, to issue a stark warning to migrants considering journeying north to the nation’s southern border: Don’t come now — but help is on the way. The next day, Biden was similarly pointed, saying his administration — while eager to roll back Donald Trump’s immigration policies — first needed to implement “guardrails” to avoid winding up with “2 million people on our border.” Less than a month later, the new president began tearing down...