A federal judge on Thursday barred the Trump administration from deporting any Venezuelans from South Texas under an 18th-century wartime law and said President Donald Trump’s invocation of it was “unlawful.”

U.S. District Court Judge Fernando Rodriguez Jr. is the first judge to rule that the Alien Enemies Act cannot be used against people who, the Republican administration claims, are gang members invading the United States. Rodriguez said he wouldn’t interfere with the government’s right to deport people in the country illegally through other means, but it could not rely on the 227-year-old law to do so.

“Neither the Court nor the parties question that the Executive Branch can direct the detention and removal of aliens who engage in criminal activity in the United States,” wrote Rodriguez, who was nominated by Trump in 2018. But, the judge said, “the President’s invocation of the AEA through the Proclamation exceeds the scope of the statute and is contrary to the plain, ordinary meaning of the statute’s terms.”

In March, Trump issued a proclamation claiming that the Venezuelan gang Tren de Aragua was invading the U.S. He said he had special powers to deport immigrants, identified by his administration as gang members, without the usual court proceedings.

“The Court concludes that the President’s invocation of the AEA through the Proclamation exceeds the scope of the statute and, as a result, is unlawful,” Rodriguez wrote.

In an interview on Fox News, Vice President JD Vance said the administration will be “aggressively appealing” the ruling and others that hem in the president’s deportation power.

“The judge doesn’t make that determination, whether the Alien Enemies Act can be deployed,” Vance said. “I think the president of the United States is the one who determines whether this country is being invaded.”

The chair of the Congressional Hispanic Caucus, Rep. Adriano Espaillat, D-N.Y., said in a statement the judge had made clear “what we all knew to be true: The Trump administration illegally used the Alien Enemies Act to deport people without due process.”

The Alien Enemies Act has only been used three times before in U.S. history, most recently during World War II, when it was cited to intern Japanese-Americans.

The proclamation triggered a flurry of litigation as the administration tried to ship migrants it claimed were gang members to a notorious prison in El Salvador.

Rodriguez’s ruling is significant because it is the first formal permanent injunction against the administration using the AEA and contends the president is misusing the law. “Congress never meant for this law to be used in this manner,” said Lee Gelernt, the ACLU lawyer who argued the case, in response to the ruling.

Rodriguez agreed, noting that the provision has only been used during the two World Wars and the War of 1812. Trump claimed Tren de Aragua was acting at the behest of the Venezuelan government, but Rodriguez found that the activities the administration accused it of did not amount to an invasion or “predatory incursion,” as the statute requires.

“The Proclamation makes no reference to and in no manner suggests that a threat exists of an organized, armed group of individuals entering the United States at the direction of Venezuela to conquer the country or assume control over a portion of the nation,” Rodriguez wrote. “Thus, the Proclamation’s language cannot be read as describing conduct that falls within the meaning of ‘invasion’ for purposes of the AEA.”

If the administration appeals, it would go first to the New Orleans-based 5th U.S. Circuit Court of Appeals. That is among the nation’s most conservative appeals courts and it also has ruled against what it saw as overreach on immigration matters by both the Obama and Biden administrations. In those cases, Democratic administrations had sought to make it easier for immigrants to remain in the U.S.