This chapter considers changing Roman attitudes and ideology in relation to the fundamental issues of war and peace. The first half examines the basis for the now common assumption that the Roman state and society had a positive attitude to warfare during the Republic. It draws attention to some important qualifications, before assessing the extent to which the same factors continued to operate in later periods. It also discusses Roman attitudes to peace, a subject that has received much less attention in modern scholarship. The second half focuses on the related themes of Roman attitudes to victory and defeat. Roman military successes during the Republic encouraged the development of a range of rituals associated with the celebration of victory (most famously the triumph) and reverence for the goddess Victoria. These rituals illuminate the close relationship between war and religion in Roman culture, while also highlighting the political implications of military success. Their evolution in subsequent periods of Roman history is considered, alongside Roman responses to defeat – how it was explained and how those explanations changed over time.