President Donald Trump’s invocation of a wartime power to summarily deport Venezuelan nationals to El Salvador was “unlawful,” a federal judge ruled Thursday, blocking the administration from further deportations under the Alien Enemies Act of 1798.

The decision from U.S. District Judge Fernando Rodriguez, Jr., a Trump appointee, is the latest sharp rebuke to one of Trump’s most aggressive and high-profile efforts to quickly carry out deportations with little or no due process.

Rodriguez said Trump had improperly invoked the law — which gives the president the power to quickly expel nationals of invading countries — to target alleged members of a Venezuelan gang, Tren de Aragua.

The Alien Enemies Act applies only when the country is facing an armed, organized attack, Rodriguez ruled. Trump’s claims about Tren de Aragua’s activities in the United States fall short of that bar, the judge added.

The act had been invoked just three times in U.S. history before Trump issued a proclamation last month deploying it against Tren de Aragua.