context...(see link)...Russia was turning U.S. and U.K. nuclear scientists starting in the early 1940s when the Manhattan Project first got started...so in those early 'Cold War' days Truman had valid reasons to be concerned...it is only with 20/20 hindsight that the unintended consequences of (Republican) Joe McCarthy's malignancy could have been foreseen.
Link: https://www.smithsonianmag.com/history/spies-who-spilled-atomic-bomb-secrets-127922660/