Microsoft Files Court Brief Supporting Anthropic Against Pentagon AI Ban

In an unusual move, Microsoft has thrown its weight behind rival AI company Anthropic, filing an amicus brief in U.S. District Court urging a judge to temporarily block the Pentagon’s designation of Anthropic as a supply chain risk. The Department of Defense banned Anthropic’s technology last week, a label historically reserved for foreign adversaries like Chinese telecom firms.

What Happened

Last week, the DOD officially banned Anthropic’s technology and deemed the company a supply chain risk. The designation, which was effective immediately, requires defense vendors and contractors to certify that they don’t use Anthropic’s Claude models in their work with the Pentagon.

Anthropic sued the Trump administration on Monday, calling the government’s actions “unprecedented and unlawful,” and claiming they are “harming Anthropic irreparably,” putting hundreds of millions of dollars worth of contracts in jeopardy.

Microsoft’s Position

Microsoft warned that without a temporary restraining order, it and other tech companies would need to “act immediately to alter existing product and contract configurations” used by the Defense Department. “This could potentially hamper U.S. warfighters at a critical point in time,” the filing said.

Microsoft invested up to $5 billion in Anthropic in November 2025 and has also been a major investor in rival OpenAI since 2019. The company said a temporary restraining order will allow Anthropic and the DOD to pursue a “negotiated resolution.”

The Bigger Picture

Anthropic had been renegotiating its contract with the DOD but talks collapsed. Anthropic sought assurance its models would not be used for fully autonomous weapons or domestic mass surveillance, while the DOD wanted unfettered access for all lawful purposes. Neither side budged.

Following the ban, Microsoft, Amazon, and Google all updated customers that Anthropic’s products outside defense work would remain accessible on their cloud platforms. Anthropic, valued at $380 billion, has become one of the fastest-growing tech startups in the U.S.

The Bottom Line

When Microsoft — which has its own massive defense contracts — publicly backs a competitor against the Pentagon, something extraordinary is happening. The DOD labeling an American AI company as a supply chain risk, using a designation meant for foreign adversaries, sets a precedent that could affect any tech company doing business with the government. The real question isn’t whether Anthropic wins this fight, but what it means for the entire AI industry’s relationship with the military.