When President Trump abruptly fired the head of the National Security Agency and U.S. Cyber Command on Thursday, it was the latest in a series of moves that have torn away at the country’s cyberdefenses just as they are confronting the most sophisticated and sustained attacks in the nation’s history.

The commander, General Timothy D. Haugh, had sat atop the enormous infrastructure of American cyberdefenses until his removal, apparently under pressure from the far-right Trump loyalist Laura Loomer. He had been among the American officials most deeply involved in pushing back on Russia, dating to his work countering Moscow’s interference in the 2016 election.

His dismissal came after weeks in which the Trump administration swept away nearly all of the government’s election-related cyberdefenses beyond the secure N.S.A. command centers at Fort Meade, Md. At the same time, the administration has shrunk much of the nation’s complex early-warning system for cyberattacks, a web through which tech firms work with the F.B.I. and intelligence agencies to protect the power grid, pipelines and telecommunications networks.

Cybersecurity experts, election officials and lawmakers — mostly Democrats but a few Republicans — have begun to raise alarms that the United States is knocking down a system that, while still full of holes, has taken a decade to build. It has pushed out some of its most experienced cyberdefenders and fired younger talent brought in to design defenses against a wave of ransomware, Chinese intrusions and vulnerabilities created by artificial intelligence.

“At a time when the United States is facing unprecedented cyberthreats — as the Salt Typhoon cyberattack from China has so clearly underscored — how does firing him make Americans any safer?” Senator Mark Warner of Virginia, the top Democrat on the Senate Intelligence Committee, said on Thursday night after General Haugh’s ouster.

Mr. Warner was referring to an operation in which Chinese intelligence bored so deeply into American telecommunications networks that it had access to the Justice Department’s system for lawful interception of calls or text messages and could listen in on some conversations, including Mr. Trump’s during his campaign last year.

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