For Jess Concepcion, a microbiology student from the Philippines, obtaining a doctorate from a university in the United States had been a dream. It was where most of his academic mentors had studied and done research, and he wanted to follow in their footsteps.

But when the United States, under President Trump, started pausing visa interviews during peak season this spring, threatening to deport international students for political speech and slashing funding for academic research, he quickly changed plans. Applications for Ph.D. programs take years and have to be tailored to specific schools, so he is aiming for programs in Switzerland and Singapore instead.

“That uncertainty made me stop in my tracks and choose another country,” Mr. Concepcion, 24, said. “Immigration policy is quite restrictive, and I’m on a different side of the world. So living in that kind of instability that far away isn’t healthy for me.”

It’s a quandary facing many young people around the world. According to the United Nations, 6.9 million people studied outside their home country in 2022. The United States has long attracted the most foreign students, 1.1 million in the 2023-24 academic year.

It’s too soon to know whether more foreign students will choose not to attend U.S. schools. But warning signs abound.