In mid-April, as he hit his stride in his first year at Harvard, Alfred Williamson felt a stirring of unease. The Trump administration’s latest threats against his university were now focused on blocking international students from attending it.

Mr. Williamson, from Wales, consulted his American friends: Should he be worried?

“They said, ‘There’s no way he would do that,’” Mr. Williamson, 20, recalled in an interview. “They said it was just a scare tactic. But they were wrong.”

Five weeks later, just after the end of the spring semester, Mr. Williamson, now at a summer abroad program, picked up his phone and saw a pileup of missed calls and messages. The threat his friends had dismissed had become a searing reality: The federal government had effectively blocked Harvard’s ability to enroll foreign students — abruptly thrusting Mr. Williamson and others into an excruciating limbo.

As anxious messages pinged on his phone, Mr. Williamson scrolled through texts from worried family members (“I hope you’re OK”) and from classmates unmoored by uncertainty (“What are we going to do?”). He spoke to one close friend who called him in tears, devastated at the potential loss of financial help from Harvard — money that had not been matched by British universities and that had put college within reach.

A judge has issued temporary restraining orders pausing the Trump administration’s efforts. But Mr. Williamson and thousands of other international students are still dogged by uncertainty, wondering what will come next.

In the country’s raging debates over immigration, someone with Mr. Williamson’s profile — a white, British man in the United States to study science — would not usually be the focus of debate. But like all kinds of students from all over the world, he, too, has been swept up in the Trump administration’s fight over higher education.