The downfall of Oppenheimer has the flavor of a Greek tragedy, but his persecutors were Americans, some naturalized and some born here. From The New York Review of Books:
The man who provided the argument and the occasion was William Liscum Borden, a single-minded young zealot who thought he knew why Oppenheimer resisted Air Force demands for hydrogen bombs—“more probably than not,” Borden wrote the head of the FBI in November 1953, “J. Robert Oppenheimer is an agent of the Soviet Union.”
But neither could have managed Oppenheimer’s destruction without the help of the obsessed H-bomb promoter Edward Teller, who had never forgiven Oppenheimer for choosing another man to run the theoretical division at Los Alamos, who dreamed of replacing Oppenheimer as the protean man of the hour, and who nursed matters forward as he methodically planted seeds of suspicion in the minds of Borden, Strauss, and Air Force generals that Oppenheimer’s “faulty judgment” could be traced to hidden loyalties from his Red period.
Borden wrote a letter which repeated many well-known and thoroughly investigated facts about Oppenheimer's links to some Communists, except that he included in each paragraph the claim that the probable explanation for those facts was that Oppenheimer was a Communist agent. Eisenhower was shown the letter, and he immediately recognized the total absence of any new or persuasive evidence, but he was also feeling the heat from Joseph McCarthy, so didn't dare object.
Interestingly, Borden had played a key role in the loss of a document containing the only real secret of the US H-bomb at that point, the radiation implosion mechanism. He had mailed a copy to John Archibald Wheeler (another H-bomb hysteric), who promptly lost it. Wheeler thought that he had left it on a train, but even partial disassembly of the Pullman car involved failed to reveal it.
The review of several Oppenheimer related books linked here and above, btw, is a great short read on the whole business.