Given the reality of mixed loyalties, Arctotherium notes, it shouldn’t be surprising that skilled immigrants often constitute an espionage risk:
Take the infamous Pakistani nuclear physicist AQ Khan. In 1961, he moved to West Berlin as a foreign student, then to the Netherlands and finally Belgium to finish his education, graduating with a Doctorate in Engineering in 1972. Khan was undoubtedly among the best and brightest of Pakistan, the sort of high-agency STEM genius that brain drain advocates hold up as America’s greatest strength. Was allowing A.Q. Khan into the West a good decision? No.
Khan got a position at the Physics Dynamics Research Laboratory, a Dutch firm specializing in uranium enrichment via centrifuge. He stole centrifuge designs and blueprints, and after returning to Pakistan set up an international network of illicit suppliers for centrifuge parts using his contacts, leading to the 1998 Pakistani nuclear bomb. From there, he diffused nuclear technology further. The North Korean, Iranian and Libyan nuclear programs all trace back to A.Q. Khan. Pakistan has had multiple serious nuclear war scares with India in the last five years. North Korea, which has a history of doing things like axe-murder Americans, can act with relative impunity thanks to its nuclear arsenal, and Israel and the US recently bombed Iran over their nuclear program.
There are many examples from the US. For instance, Noshir Gowadia, an Indian Parsi designer of the B-2 Spirit stealth bomber, and Chi Mak, who worked on nuclear submarines, both sold secrets to China.