During the final months of the Biden presidency, a federal appeals court judge made what was then a startling assertion: The number of undocumented immigrants coming across the southern border amounted to an “invasion.”

The assertion that the United States was under attack appeared in a partial dissent by Judge James C. Ho of the U.S. Court of Appeals for the Fifth Circuit in a dispute over the legality of buoys that Gov. Greg Abbott of Texas had placed along the Rio Grande. Framing migrants as invaders was by then a familiar trope in the conservative media ecosystem and in the campaign rhetoric of then-candidate Donald J. Trump.

But Judge Ho’s opinion marked the first time a federal judge had put a legal imprimatur on the concept of a migrant “invasion,” giving new legitimacy to a theory Mr. Trump would attempt to use to transform long-established understandings of due process for migrants.

In the case, Mr. Abbott, a Republican, had used it to justify placing the buoys along the Rio Grande by invoking his state’s constitutional power to “engage in war” if “actually invaded.”

Ten appellate judges agreed with Mr. Abbott about the buoys, including all six of Mr. Trump’s nominees to the stridently conservative Fifth Circuit. But they stopped short of endorsing the invasion theory, except for Judge Ho.