Henry Rousso, a French historian and one of the most pre-eminent scholars on the Holocaust, said he was detained for more than 10 hours by federal border agents in Houston and told he would not be allowed to enter the United States before lawyers intervened to stop his deportation.
Mr. Rousso said in a telephone interview on Sunday that he arrived at the George Bush Intercontinental Airport around 2 p.m. Wednesday on a flight from France when immigration authorities began to question his visa and his reason for being in the United States.
Mr. Rousso, an expert on France after the First World War, was scheduled to give a keynote address on Friday afternoon at a conference organized by the Hagler Institute for Advanced Study at Texas A&M University in College Station.
“It would be in no means difficult to look up who he is,” said Jason Mills, an immigration lawyer who helped secure Mr. Rousso’s eventual release. “His reasons for being here were nothing but beneficial to the United States. He is a man of experience and age,” Mr. Mills said. “There is plenty of history there on him. I don’t understand why he would have been in for the several hours that he was. It is a little alarming.”
Mr. Rousso said he was interrogated by Customs and Border Protection officers who told him that he was violating immigration law by using a tourist visa to enter the country to attend the academic conference. He said that at first they denied him entry to the United States, and told him he would be put on the next available flight to Paris.
The academics who had invited Mr. Rousso to speak in Texas became concerned when he failed to meet the driver who had been sent to collect him. They scrambled to alert immigration lawyers, the dean of the law school and Michael Young, the president of Texas A&M University.
The issue, Mr. Rousso said, appeared to be an honorarium of $2,000 that he was being paid to participate in the conference. Such payments are allowed for academics visiting the United States, but Mr. Rousso and those involved in securing his release said the customs agents appeared not to realize that at first.
“With a tourist visa, I’m not allowed to work,” Mr. Rousso said. “This is true — except for scholars.”
The agent who was questioning Mr. Rousso was “concerned that he was giving a lecture and was getting a good stipend to do that,” said Richard J. Golsan, a professor at the university who also had planned to have Mr. Rousso speak to his class last week.
Customs and Border Protection did not respond to a telephone message or email requests for comment on Sunday.
He set to work contacting immigration authorities at the Houston airport.
It was after 1 a.m. Thursday when Mr. Rousso was given back his passport and cellphone, taken to a public area of the airport and told he was free to go. He said he was told that the agent who originally held him was “inexperienced.”
He took a taxi to an airport hotel, where he was able to telephone Mr. Golsan, and to continue his journey to the university.
He gave his keynote address, “Writing on the Dark Side of the Recent Past,” as planned on Friday. On Sunday morning, a few hours before he was to board a flight to Paris, Mr. Rousso, 62, said in the telephone interview that he was apprehensive about returning to the airport. He has for 30 years been a regular visitor to the United States, and was unsure when he would return, he said.
“I’m a little bit nervous,” he said. “It’s completely irrational, I know.”
Mr. Mills said the treatment Mr. Rousso experienced was unusual, but representative of a shift in how some border agents are approaching their jobs.
“Now they’re looking really hard for reasons to deny, instead of reasons to admit,” he said.
Mr. Rousso and those who helped him said he was lucky to have been able to reach out to leaders at the university.
“If I had not the possibility to call my friend and then to be in touch with the president, probably I would have been in Paris now after a bit of blurry, strange experience,” he said.
In France, where Mr. Rousso is a well-respected academic, his treatment was met with anger.
Emmanuel Macron, a centrist candidate for France’s presidency, condemned the episode on Sunday on Twitter, saying there was “no excuse” for what happened to Mr. Rousso.
Fatma E. Marouf, a law professor and the director of the Immigrant Rights Clinic in Fort Worth, who helped secure Mr. Rousso’s release, said he benefited from the lessons that immigration lawyers learned in January, after an executive order from President Trump led to chaos at the nation’s borders. “During the airport detentions, we had created a really good rapid-response team of attorneys in Houston and where I am in Dallas-Fort Worth,” she said. “There was already a good team in place.”
Since Mr. Trump took office in January, immigration authorities have engaged in several high-profile actions against immigrants. Sean Spicer, the White House press secretary, said on Tuesday that the president wanted to “take the shackles off” of agents who had, under President Barack Obama, been under orders to focus only on serious criminals.