fifth columnist

MURDER at the Tokyo American Clubの第１０章は１９２８年に設立された東京アメリカンクラブの歴史をたどっていて、次ぎの文は１９３０年代後半の頃の状況について書いてある個所です。
Later, suspicions that the club harbored spies, malcontents, revolutionaries, anti-militarists, seditionist, oppressors, fifth columnists, appeasers, constitutional democrats, and folks without a sense of humor about world affairs had gradually diminished the pleasure of the dining and social intercourse.
"fifth columnists" は全く見当がつかない表現なので早速辞書を見ます。
・Dictionary.com:
1. a group of people who act traitorously and subversively out of a secret sympathy with an enemy of their country.
2. (originally) Franco sympathizers in Madrid during the spanish civil war: so called in allusion to a statement in 1936 that the insurgents had four columns marching on Madrid and a fifth column of sympathizers in the city ready to rise and betray it.
・Vocabulary.com: a member of a clandestine subversive organization who tries to help a potential invader: On the web, the journalists are denounced as "traitors" and "fifth columnists.": "If you are fifth columnists," he said, "I spit on your cigarettes."