The Bureau of Alcohol, Tobacco, Firearms and Explosives — a small government agency tasked with the titanic challenge of stemming the spread of illegal guns — has long been regarded as the spurned stepchild of federal law enforcement.

It is fast becoming an orphan.

The bureau, responsible for enforcing gun laws and regulating firearms dealers, enjoyed a brief but consequential revival during the Biden administration. It led a four-year push to expand background checks on buyers, crack down on untraceable homemade weapons known as “ghost guns” and curtail the use of devices that convert standard weapons into machine guns.

That ended when President Trump took office. Since then, the A.T.F., a division of the Justice Department, has been ravaged by the departure of key career personnel, the diversion of dozens of agents from core duties to immigration enforcement and from what amounts to a campaign of indifference — leaving it rudderless, leaderless and demoralized.

On Wednesday morning, the A.T.F.’s roughly 10,000 employees were handed off from a distracted caretaker, the F.B.I. director Kash Patel, and dumped on the bureaucratic doorstep of the Army secretary. The secretary, Daniel Driscoll, had only been told he was being saddled with the assignment a few days before, according to people familiar with the situation who spoke on the condition of anonymity to publicly discuss the matter.

The highly unusual move placed a civilian military leader in charge of a domestic law enforcement entity for the first time in memory, and critics of the administration were quick to discern sinister motives.

The truth was less menacing, if no less damning for an agency Republicans have long sought to handcuff and marginalize: Mr. Patel was too busy doing his day job, and Mr. Driscoll had been confirmed by the Senate, a prerequisite for taking over.