Final answer:
American concern over Soviet spies was rooted in the heightened Cold War tensions, confirmed cases of espionage, and fears of national security breaches.
Step-by-step explanation:
Americans were concerned about Soviet spies because of the tense geopolitical climate and the revelations of actual espionage during the post-World War II era, known as the Second Red Scare. The presence of confirmed services such as those by State Department officials and the Manhattan Project espionage, alongside high-profile accusations such as those against Alger Hiss, fueled public anxiety. Incidents like the U-2 Affair and the execution of the Rosenbergs for espionage, coupled with Senator McCarthy's allegations of widespread communist infiltration, deepened fears of Soviet spies compromising U.S. national security.
Moreover, the victory of Communist forces in China, the outbreak of the Korean War, and the political climate that emerged within the United States created a situation where political leaders did not want to be seen as soft on Communism. These factors contributed to the fear that Soviet spies may be undermining national security and working against American interests.