The article argues private information plays an important role in explaining long wars. Existing rationalist explanations of long, intensely fought wars focus on commitment problems rather than private information as the cause of such wars. Commitment problem explanations of long wars claim that while private information can explain short wars, battles and exchanges of offers for settlement should quickly reveal private information thereby leading to an early peace. Commitment problems, on the other hand, may take years to resolve and therefore can explain long, intense wars for unitary actors. However, while commitment problems are an important explanation for long wars, private information can endure deep into lengthy conflicts because new private information is created during wars and because states often disagree about their relative ability to bear costs rather than their relative military capabilities. This argument is explored in cases on the end of the First World War and the Iran-Iraq War.