The administration is veering wildly in its response to the national security implications of A.I. as China, America’s main technological and military rival, charges ahead.

In mid-July, many of the country’s biggest military contractors received a letter from the Air Force with a stern warning: By Sept. 1, all the software they use for weapons and control systems must be free of products built by the A.I. company Anthropic. Failure to comply would put all their business with the Pentagon at risk.

Within a month, those same contractors received an unexpected reversal: They could — for now — disregard the earlier instructions about purging Anthropic.

It was one more example of the chaos and contradictions in the tsunami of A.I. disruptions that have swept through the national security establishment in recent months. Agencies like the Pentagon, which still use airplanes designed in the 1950s, are struggling furiously to keep up with A.I. models whose powers multiply in a matter of months.

The tension has been magnified in the case of Anthropic, whose chief executive, Dario Amodei, has been locked in a public feud with Defense Secretary Pete Hegseth over restrictions the company tried to put on the technology’s use by the world’s most powerful military.

Without Anthropic’s powerful Mythos model, it would be far more difficult for the National Security Agency to find and patch vulnerabilities in the Pentagon’s own systems, senior N.S.A. officials argued. And it turned out that, despite the original Anthropic ban, the N.S.A. was still allowed to use its products.

Critical tests were underway using Mythos to gauge its potential to hack into highly protected foreign networks, current and former officials said. Shutting down the use of Mythos, an A.I. model whose potential for conducting offensive cyberoperations is already shifting the digital arms race into overdrive, would have amounted to “unilateral disarmament,” according to one recently departed senior U.S. official.

From the Department of Defense to the C.I.A. and N.S.A., billions have already been spent on using artificial intelligence to develop weapons that could work with less human control, or to test vulnerable military networks and even nuclear codes to make sure they cannot be cracked by adversarial A.I. systems. Artificial intelligence is critical to the Golden Dome, President Trump’s vision — fanciful to many experts — of a space-based missile shield.

But little of that work anticipated the powers of Mythos and its A.I. counterparts, or the prospect that China may be months away from its own formidable and less expensive versions. That could supercharge China’s ability to infiltrate American communications networks, water systems and power grids like those they successfully pierced during the Biden administration in two sweeping attacks called “Volt Typhoon” and “Salt Typhoon.”

“These are the kinds of things we were talking about and worrying about before A.I. became a daily headline,” said Jen Easterly, who ran the Cybersecurity and Infrastructure Security Agency during the Biden administration. “What’s different now, and what makes the problem more urgent, is the potential for A.I. to amplify those capabilities.”

So far the Trump administration has veered wildly in response, at first abandoning its hands-off approach to regulating the industry, then briefly shutting off access for foreigners from Mythos — including some of its inventors — then lifting that ban.

Earlier this month, the administration called in executives from the largest “frontier” creators, including OpenAI and Anthropic, and told them that new models would be reviewed by the government to assure they could not be used to make biological weapons, or break into classified American systems or the communications networks that control nuclear weapons.

(The New York Times has sued OpenAI and Microsoft, claiming copyright infringement of news content related to A.I. systems. The two companies have denied those claims.)

But the government review process exempts “open” models — including many of those now produced by Chinese firms — whose code can be customized by users.

One senior executive of a company briefed on the new policy said it made no sense, noting that the open models can be just as dangerous. Like many in the industry, the executive would not be quoted, for fear of angering administration officials.

Pentagon officials reject the idea they are not agile enough. They say the U.S. military has maintained immediate access to the world’s most advanced A.I. systems, despite the Anthropic ban. An official, who discussed Pentagon policies on the condition of anonymity, said the Defense Department had recently integrated a version of a Google Gemini model within four days of its commercial release.

More than 1,300 A.I. workers have called for the country to slow development. But others, including inside the Trump administration, have answered that call with a question: “What about China?” They fear losing the roughly six-month lead they believe the United States maintains.