Final answer:
The author uses historical evidence, including the Alger Hiss case, ancient military strategies, and Cold War espionage actions by the CIA and KGB, to support the claim about defectors becoming foreign agents.
Step-by-step explanation:
The author uses historical evidence to support the claim that defectors were spies in their own country but then became agents for a foreign government. One notable case cited is that of Alger Hiss, a former State Department official accused by Whittaker Chambers, a former communist spy, of spying for the Soviet Union. Another form of evidence provided includes the strategies documented in ancient military treatises, such as the use of converted spies, as described in the excerpts provided, showing how defectors could be integral to espionage efforts.