From Longman Dictionary of Contemporary Englishthe enemy withinthe enemy withinpeople in a society etc that other people think are trying to secretly destroy or damage it efforts to label environmentalists as the enemy within → enemy
Examples from the Corpus
the enemy within• But he's now an associate fellow at Templeton College in Oxford ... so is he viewed as the enemy within.• Chapter 1 focuses on current efforts to label the poor, specifically poor women, as the enemy within. 