Why was Qian Xuesen detained by the United States?

In 1949, when news of the founding of New China reached the United States, Qian Xuesen and his wife Jiang Ying could not restrain their joy and discussed returning to China as soon as possible. At that time, there was a hysterical fanatic movement in the United States that drove employees to be loyal to the American Communist Party. Because he was suspected of being a communist and refused to expose his friends, Qian Xuesen was suddenly revoked by the U.S. military department from his certificate to participate in classified work. Qian Xuesen felt very angry. He said, "I would rather go back to my hometown in China than continue to live in the United States under suspicion!" But what he never expected was that his long-cherished wish to return to China would actually come true. A disaster. After the U.S. Under Secretary of the Navy heard the news that Qian Xuesen was returning to China, he said viciously: "He knows the core secrets of all U.S. missile projects. One Qian Xuesen is worth five Marine divisions. I would rather shoot this guy than let him go." Go back to Red China!” After that, the Immigration Bureau raided Qian Xuesen’s home and imprisoned him on Termina Island.