Misinformation remains a persistent global challenge in modern media systems. We investigate whether and under what conditions fact-checking effectively deters misinformation and promotes truth, focusing on the strategic interaction where a biased outlet decides whether to misreport and a fact-checker decides whether to scrutinize the report. Our findings suggest that the deterrent effect of fact-checking hinges critically on the degree of media market polarization. Provided that the market is not excessively polarized, fact-checking strictly reduces misreporting by biased outlets, thereby improving information welfare---a result that echoes the classical views of John Milton and John Stuart Mill that the competition between opposing claims facilitates the discovery of truth. We also show that the presence of an active fact-checker can make biased outlets better off by serving as an external commitment device, thereby resolving the dynamic inconsistency problem.