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So, you actually think it was “illegal” for every wartime President to spy on the enemy within our own borders, huh
No, not until 1978, when it became law that they obtain a warrant as required by the Foreign Intelligence Surveillance Act passed that year -- including in time of war. That is, if by "enemy" you mean "U.S. persons" as defined by FISA.
Altho, even prior to that such activity may have been unconstitutional; that remains an unsettled question.