What does "When a democracy is destroyed, it will not be from enemies from without, but rather because of enemies from within" mean? Why might Senator McCarthy have chosen to use this quote to support his claims?

"When a democracy is destroyed, it will not be from enemies from without, but rather because of enemies from within" means that Senator Joseph McCarthy was referring to the suspected infiltration of the Communists into the US Armed Forces.

Why might Senator McCarthy have chosen to use this quote to support his claims?

He chose to use this quote to support his claims of anticommunist hysteria of the Red Scare in America. He believed without substantial evidences that the US armed Forces has been infiltrated by the communist loyalist and if allowed unchecked could destroy their most cherished democracy. Unfortunately, all these came to ruins when he was shut down from his bogus fame when the soft spoke lawyer, with incisive wit and intelligence, Joseph Welch, special counsel for the U. S. Army, in a dramatic confrontation of the defense of the Armed forces in his celebrated statement "Until this moment, Senator, I think I never really gauged your cruelty or your recklessness."

The qoute by Senator McCarthy is all about the gains of democracy being eroded by party members of the same party as a result of the individual ambitions of the party members rather than the common goal of the party.

He might have chosen to use this quote to support his claims as a result of the fact that he suspected that the then president (President Truman) harbours communists that are well known in the government circles. This qoute is part of the speech he delivered as a result of him being suspected and accussed of being disloyal to the country.

During the speech, he presented the copy he has as the list of people he accused of being communist even though he never made their names public.