Just as he had on previous trips, Zhu Feng bolted down his lunch at a Los Angeles airport before sprinting to catch his Air China flight back to Beijing.

Suddenly, two F.B.I. agents blocked the Chinese scholar at the boarding gate and ordered him to hand over his passport. They flipped to the well-used 10-year visa to the United States and crossed out the page with a black pen.

“‘Go back to China,’” Mr. Zhu, a professor of international relations, recalled an agent telling him during that visit in January last year. “You will receive a notification.”

In the four decades since China and the United States normalized relations, Washington has generally welcomed Chinese scholars and researchers to America, even when Beijing has been less open to reciprocal visits. Republican and Democratic administrations have operated on the assumption that the national interest was well served by exposing Chinese academics to American values.